Necrophos

Rotund'jere the Necrolyte is a ranged intelligence hero who typically plays as a mid-game carry. His skills are most effective in team fights where he can damage enemies and heal allies simultaneously, while picking off key enemy heroes with his ultimate. Necrolyte is most dangerous when his enemy is severely injured, instantly killing them with his ultimate while recovering over time after killing his foe. Necrolyte is naturally fragile, but his mechanics require him to stay in the midst of encounters; it is for this reason that he depends on items to prevent his death. Ideally, by casting Death Pulse repeatedly, he and his team are able to stay alive while the enemy team's health is gradually sapped by Heartstopper Aura, which reduces enemy HP by a small percentage each second. Because his abilities are suited to prolonged encounters, he must build items which allow him to survive for as long as possible against his enemies. His ultimate, Reaper's Scythe, has its damage increased by how much of their maximum HP the target is missing; meaning that an enemy that is close to death will be killed outright by it. Upon attaining a kill, Sadist restores Necrolyte's mana and HP over a short time, allowing him to continue fulfilling his role in encounters. Necrolyte is best understood as a hero who is weak at the beginning of a fight but becomes more dangerous with each passing second.

Tips

 * Taking one point in Heartstopper Aura in the early game allows Necrolyte to harass enemy heroes by simply standing near them.
 * Note however that the aura affects enemy creeps, and thus putting points in it will push the lane. Should you lane with a farm-dependent carry, it is unwise to push as it makes their farm more difficult and leaves both of you more vulnerable to ganks. Putting one or at the most two points in it is appropriate for early game.
 * The philosophy behind Reaper's Scythe is that (owing to Sadism) it will return its mana expenditure if you secure a kill. There are several situations in which this ability should be used purely for its stun, such as interrupting channelled ultimates (such as those belonging to Pudge or Enigma) or preventing an enemy hero from escaping. Remember that the stun goes through magic immunity, although the damage will not.
 * Reaper's Scythe will almost definitely kill targets at 35% or less hp (with Aghanim's Scepter it's generally any amount under 45%). Remember to factor in Magic Resistance before you attempt to execute an enemy hero with it.
 * Aghanim's Scepter upgrades Necrolyte's ultimate and will increase the damage ratio to 1.2 at max rank. To calculate the raw damage of Reaper's Scythe, multiply the missing health of an enemy by the ratio. For example, if Axe has a maximum health of 3,900 and he has lost 2,200 health, multiply 2,200 by 1.2 and the damage of Reaper's Scythe will be 2640, (ignoring Magic Resistance), which is more than lethal enough to kill him.
 * Note that if someone else kills the target of this spell in the brief delay between the stun and the scythe, the kill will be theirs. For this reason it is unwise to use it on an enemy hero that you know will die regardless.
 * The Reaper's Scythe instant-kill HP threshold, with the base 25% magic resist factored in and without Aghanim's Scepter, can be calculated by multiplying the enemy max health with the constants: 3/13, 9/29, or 27/67 for each respective level of the ultimate. With the Scepter, the values change to 9/29, 27/67, and 9/19. Example: If Windrunner has 700 max health; 700*3/13 = 161. Id est, she must have 161 or less HP to be killed by level 1 Reaper's Scythe without Scepter.
 * Necrolyte can very easily be built as a tank and in most circumstances should be; you may gear for Strength (he does not need excessive Intelligence as his autoattack is not useful and his spells are not particularly expensive, besides which Sadism refunds mana) and consider a Vanguard instead of or in addition to your Mekansm. The reason he is usually built as a tank is because he needs to stay alive in violent encounters to heal his allies (a good enemy team will focus him first), and also because the longer he is alive for, the more damage his aura will do and the better chance he has of landing a crucial Reaper's Scythe.
 * On the topic of items, a Dagon can prove effective if the early gold is available, providing Necrolyte with more Intelligence as well as a means to supplement his lack of burst damage. The extra spell cast will allow him to more effectively use Reaper's Scythe for securing kills. Upgrading this item fully is not advisable, as excess gold could be better spent on utility items like Force Staff or Mekansm. Utility items are more useful for Necrolyte than for most heroes, as because of Heartstopper Aura he can inflict constant damage simply by avoiding death.
 * Necrolyte is effective against invisible heroes that tend to stalk their prey as his Heartstopper Aura does damage while they are in stealth, and his Reaper's Scythe will prevent them from escaping via invisibility before they die. He is also very good against heroes with abilities that deplete their own HP, such as Pudge and Huskar, but Necrolyte should first be durable enough to take on these kinds of risk/reward heroes.
 * Necrolyte's role is notoriously debated; while his abilities appear supportive at their core, his Reaper's Scythe is a tool used to secure kills only for himself (much like Axe's Culling Blade). This is atypical for a standard support hero, because supports usually attempt to confer gold and experience to the team's hard carry. The duplicity of his carry/support role stems from his unique stance in encounters; his kit is largely supportive until an enemy hero grows weak, at which point Necrolyte has the capacity to kill said enemy himself. He is best viewed as a potent support unit, who becomes gradually more dangerous as a fight goes on.
 * Necrolyte is countered by any hero with high HP regeneration, such as Dragon Knight and Quas-focused Invoker, or a hero with lifesteal capabilities; because he relies on inflicting constant, gradual damage with Death Pulse and Heartstopper Aura. Heroes with these attributes will resist this in lane against Necrolyte, keeping high enough HP to prevent death by Reaper's Scythe.

Trivia

 * Necrolyte's Alternate/Fun name was "A Phoe Gyi", which means "an old man" in Myanmar language.