I've seen more and more dota blogs popping up recently, including a few hero guides, so I thought i'd contribute my own.  The following is a compilation of a few years playing my favorite hero (okay, Sand King is up there too), research done online, replays watched, and just general experience.  I mostly pub / TDA (whats the difference, right!) nowadays, but I used to play in a few leagues (mostly DXD) a while back.  I still loosely follow the competitive dota scene, so take this for what you will.

Lion, the Demon Witch

Role: Gank / Support

Strength:       16 + 1.7 / level

Agility:          15 + 1.5 / level

Intelligence:  22 + 3 / level

The Basics:

I was originally going to include a section outlining each skill etc for reference in the rest of the article, but when i went searching for the information i found this, which is better put together than anything i could.

Lion, The Demon Witch: Basic Stats and Skills

Skill Build:

This is pretty much the only build you should be going, with one variation: getting rank 4 mana drain before ranking hex at all.  Both of those builds work fine, I use both of them, its entirely game dependent though, for instance:

In a game against a lot of heroes with low mana pools but high presence skills, like Centaur, Axe, Tiny, Vengeful Spirit, Sand King, Skeleton King, Tidehunter...theres a lot of them -- I usually max mana drain first. (or in sand kings case, get 1 rank of voodoo then max mana drain)  This is because midgame, you can literally sap their entire mana pool in one impale->drain, and that gives a HUGE advantage to your team.

In terms of providing mana to yourself, its not crucial to max mana drain immediately because ranged creeps, which provide you your mana over half the time, only have 300 mana.  Rank3 drain over 5 seconds drains all of their mana, as well as a 3 second rank4 drain.  The net result is 300 mana to you either way.  Overall i'd say I split the games 50/50 whether i max drain quickly or rank 2 ranks of hex first.

Starting Items:

Being a hero with a virtually unlimited mana pool, as long as you maintain your health you should never really be headed back to base much.  This makes him a great hero to courier with, as the lack of 200 starting gold doesn’t really affect him much.  If you need additional regen or stats, its easy to bring a gauntlet or salves from base, especially with the courier changes. This is usually the build I go.  The weakness of this build is a lack of base stats can hamper how effective you can be controlling a lane against another strong lane.

 

Very defensive starting build, this gives you +8str and easily turns into 2 bracers.  If the other team has a lot of strong nukers or heroes you will inevitably tank damage from while laning (orbwalkers, nuke spammers) this is a good build to go.  Works best if someone else on your team gets a courier, as 3 tangos probably wont last.

 

Netting both +4 strength and +4 int, you get decent survivability with a nice boost for last hitting.  Also leads well into mekansm.  Most games I find the courier to be a more useful investment than 3 branches, though, as you can bring whatever early game items you need to adapt to your lane from base.  For instance, when soloing mid against a zeus with bottle,  If you get unlucky stealing runes from him you can end up back at base in no time from being overpowered by boltspam.  Using a courier to bring salves from base is a good way to hold your ground.

 

Other Possible Items

There have been guides in the past advocating stout shield, for easier tanking of creeps during an impale+drain.  While I don't disagree with it, as its a very strong item for 250g, I usually don't work it into any of my builds, because there usually isn't a lack of opportunities to impale from behind your own creep wave.

 

Starting items NOT to get:

Bottle:  You have mana drain, and you need levels quickly.  Hunting down runes cuts your xp and you’d be stealing them for heroes who need them more than you.

Boots of Speed: An investment that should be made on your first trip back to base (or through courier), but never as a starting item.

Ring of Basilius:  The mana regen is useless to you, and unless you have a very specific strategy with the hero you are laning with, this item is better bought on someone else.

Blades of Attack:  Beyond being available at the goblin merchant, the +9 damage for last hitting is nice but you are better just buying them a few minutes in than risking going back to town from lack of stats or regen.

 

Early/Mid-Game Items

            Before I continue, let me make something clear:  Lion is not a carry.  If you spend most of the game farming, sure, you may end up with your aganhim’s scepter, your guinsoo and your boots of travel.  Hell, you may even win a few of those games.  But most of them you won’t, because by the time you hit the 30 minute mark, all those agi and str carries you were busy not stifling will have their bkb and/or damage items, and you will watch in sadness as you fall over if someone like lycan or rikkimaru looks at you wrong.  That said, you also shouldn’t be at a terrible lack for money like say, crystal maiden, as impale is a wonderful tool for farming the times that are available to you, and of course the money you will inevitably get from hero kills.  Lion really doesn’t have a “core” midgame item besides wards;  the flow of the game and enemy/allied hero picks determines what you build.  Below is a list of items that work well on him;

 

Observer/Sentry Wards: (200g)  Lion is perfectly suited to the role of warding for your team, as 90% of his effectiveness as a hero is not item dependent.

 

Bracer: (510g) Bracers are a cheap, effective way to bolster your below average hit points, and get a little more base mana.  You should only be buying 1-2 of these—if you need more hp, look to a point booster, as you need free item slots and bracers take up a lot of room for a moderate gain.

 

Phase Boots: (1550g)  Phase boots are the cheapest of the upgraded boots (1550g for phase, 1850g for power treads, 2700g for travels) – and also the fastest.  Phase boots not only allow you to pass through units, activating them also increases your speed past that of boots of travel for 6 seconds on a 12 second cooldown.  70 base speed also makes them faster than 60 speed power treads 100% of the time; add in his lack of benefit from the attack speed on power treads and they can be eliminated from the argument easily.  A little extra armor and +dmg for last hitting never hurt anyone either; In the end its almost always a better decision to get phase boots and a few tp scrolls than it is to buy the more expensive boots of travel.  You should end up making these pretty much every game, upgrading your boots to these or travels really isn't optional.

 

Force Staff: (2200g)  My personal favorite item in the game right now, and I pretty much make one every game, for better or worse.  Position is the most important thing in dota, and being able to change an allied or enemy hero’s position at will is a powerful resource.  An activation cost of 100 is pretty steep for most heroes, but Lion can take it in stride due to Mana Drain.  Play around with it, every game I use it I find a new way to apply it.


Mekansm: (2431g)  A great support item, Mekansm heroes are few and far between, due to the high activation cost for an early game item (150mana)  Lion, once again, can support this because of Mana Drain.  a 250aoe heal + 2 armor is pretty huge, giving Lion some more surivivability, in addition to the +6 stats and additional regen.  Not an item I make every game by any means, but its an option.

Eul’s Staff of Divinity: (2925g)  Eul’s does a few things for lion;  20 additional speed makes him very difficult to catch, and a very good chaser, especially with phase boots.  It gets him some % based mana regen, and some additional int.  Most importantly though, it provides an already cc-heavy hero with even more cc; and everyone likes more cc.  Recently added in 6.60 is the ability to cast eul’s on yourself, the icing on the cake.  More than once I have dodged a storm bolt or hellfire blast by euls’ing myself and avoided a lethal damage train that would have followed.  A very solid item, though a little pricey.

 

Late Game Items

You probably won't end up finishing these most games, but if you're finding yourself with a surplus of gold through strong laning, lucky last hits or for whatever reason, these are the items you should be looking at:

Aghanim's Scepter: (4300g)  A great, all around item, this improves Lion's ultimate to a whopping 1025 damage on a 55 second cooldown, while giving him a huge boost to mana and hp.  Much easier to make than the old Agh's, this is a fine option, though I prefer the next choice most of the time.

Guinsoo's Scythe of Vyse: (5675) Expensive as hell, but worth it.  Guinsoos gives you base stats and regen like Aghanims, in addition to a 3 second hex on a 35second cooldown.  If you weren't a primary target before, you most certainly will be after making this.

Techniques

There are some important techniques you must master in order to play Lion well, and some certain things you need to know about his spells.  Lets start with Impale:

Impale is a targeted stun, but in addition to being able to target an enemy hero and cast it, you can Groundcast Impale by targeting the ground.  This is an important skill, becuase in some situations groundcasting Impale will give you the additional range you need to hit an enemy hero.  Below is an example of a targeted cast impale, and the additional distance that can be achieved through groundcasting:



The impale was cast by targeting Earthshaker, and was cast at the maximum range it can be cast from if you target a unit.  Notice how it hit Abbadon a significant distance behind him:  If Earthshaker wasn't there, I would have had to run forward to cast on Abbadon if i had targeted him;  the other option is to click the ground in the direction of Abbadon and i could hit him with it from where I was standing.  This is an important skill to have, as it gives you a huge advantage when laning against other stunners with targeted stuns.
Additional benefits of Groundcasting include hitting invisible units, and hitting units that have entered fog of war.  The ability to Groundcast makes Impale a top tier targeted stun.

Another thing you may have noticed was the Mana drain on Earthshaker.  It begins before the impale even hits him: this is another important skill to master.  You can cast your next spell as soon as the impale animation begins; effectively allowing you to start mana draining before your stun even hits, maximizing your drain time.  This can be achieved in 2 ways:

1) You can queue the mana drain by standing out of range of your target, casting impale by targeting your target, then, while your hero is running forward to cast impale, hold down shift while you cast mana drain on your target and it will fire immediately after your impale goes off.  The problem with this though, is that you can only cast via target; it is very difficult to grouncast in this manner.  The more usable method is to:

2) Just be really fast with casting mana drain.  Its entirely possible to cast mana drain before the impale hits by just casting it quickly.  This is how i cast it in the video above.  Practice makes perfect

Applications of these skills on their own are great, but you can make it even better.  If you use impale properly, you can get a creep kill or two, stun your oppoenent and drain his mana all in the same two spells.  Here is an example of efficient impale usage:  (I know my mana drain was a bit late, sue me)  Notice how i move to the left to get that melee creep in the impale cast for some extra gold.  Also, clinkz was too far away to cast on by target, this was a groundcasted impale, and it would not have been possible without groundcasting.

Strategy

As i've mentioned before, Lion is a fantastic gank hero, and in an average game you'll spend most of your time looking for someone to kill.  Lion is not, however, a level 2-3 ganker like some other heroes (sand king, vengeful spirit come to mind) as his stun duration increases with level, and he doesn't have a damaging supporting skill until 6.  This is not a problem though, as Lion is equally as strong in a lane as he is ganking.

One thing I will say is, if your Finger of Death is ready, you should be looking for someone to kill with it.  It is strongest at level 6 and only decreases in power as the game goes on.  500 base damage at 6 makes it the hardest hitting skill in the game at that point; make use of it.  I could write a guide on ganking but it would be much too long for this post, so i'm going to keep it short:

Rule 1:  You should almost always initiate for your gank.  Impale is a targeted stun with a relatively short recast, making it perfect for setting up a second aoe target stun or whatever follows based on the heroes you are ganking with.  The only situation you shouldn't really initiate is if another hero has a targeted stun with a shorter recast than yours (12 seconds)

Rule 2:  Dont use finger until it is necessary.  Finger of Death has a HUGE range, and you want to autoattack until your enemy is about to enter fog or is too far away to catch before attempting to finish with finger.  Below is an example of utilizing this huge range to finish off Necrolyte as he is escaping our gank:



Rule 3: Mana Drain.  There is a time and a place for mana drain in ganks, and its not every gank.  For instance, if you're ganking an antimage with 100 mana, you absolutely must mana drain him during impale to get him below his 60 mana blink cost.  However, if you're about to kill a Crystal Maiden with 500 mana and you've only got a rank 2 drain, you're not going to bring her out of her comfort zone for casting crystal nova and ice block, so don't bother and kill her as fast as possible.  Its important to get a feel for this, as it can make or break a gank.


Well, that about sums up this guide as I intend to post it here, was a bit long, but thanks for reading!  Feel free to post any additional questions in the comments!