Ostarion the Skeleton King is a melee strength hero who can be either built to tank or deal damage. He can be very tough to kill, twice.
His abilities synergise extremely well with either of the two roles that he may be given in his team and the fact that the enemy team needs to kill him twice to really get rid of him technically makes your team have 6 members instead of 5.
His first ability Hellfire Blast is a single-target stunning spell. At Level 4, it will deal 200 initial damage, stun for 2 seconds and deal an additional 50 damage per second for a 2-second duration. Following the stun, the enemy target will have their movement speed reduced by 15% for 2 seconds after. The skill has cool-down of mere 8 seconds, meaning it makes it extremely tough for the enemy target to get away from Skeleton King once and if the pursuit begins. It uses 140 mana, which by his standards is a lot. So it can pretty much hinder him from spamming the spell to his liking.
Skeleton King’s second ability is named Vampiric Aura and it is one of the many passive skills that he possesses. At Level 4, ally melee units in a 900 Area of Effect (AoE) will gain 30% life-steal from their attacks. The great thing about this life-steal is that it does not count as an orb effect. What this basically means is that this life-steal can stack with an orb effect and even life-steal. As he relies on his physical attacks to deal damage to his foes, a Desolator can come in real handy, especially considering the fact that it will stack with life-steal.
His third skill is named Critical Strike and as you can guess, it too is a passive ability. It gives him a percentage chance to do a critical strike. At its highest level, Critical Strike will cause Skeleton King to deal 2.75 times the normal damage for 15% of the time.
Finally, we get down to his fourth passive skill which also happens to be his ultimate, Reincarnation. It basically gives him a second life. Although it is a passive ability, he needs mana to use it. If you die while having 140 mana, you will revive with full health and mana after 3 seconds, while slowing enemies in a 700 AoE by 30% for 4 seconds. If you do not have the required mana or it is on cool-down when you take the killing blow, you stay buried. However, if you do have the required mana and the skill is not on cool-down but your enemy has silenced you, you will still revive. Another thing to remember is that if you are carrying Aegis and you die, your ultimate would trigger first.