Drain Blood is an ability gained by taking the "Vampiric Ability" feat. It does nether damage based on your dexterity, with nether damage's effect being life drain, thus "sucking life" and increasing your health by the amount gained. It is a useful ability for hitting monsters with high evasion early game because it cannot miss, but as most high level monsters have nether resistance (and all others but magic) at superb, hypnotism is generally a better choice if the player wants to hit a high evasion enemy. For the same reason, you can't use Drain Blood as a poor man's heal later on because it will do very little damage after the monster's resistances are applied.
The main benefit of Drain Blood in Elona+ is that it drains more stamina than it consumes, allowing you to consume most of your stamina then quickly regain it (faster than resting, if you have a target available), especially useful for spamming cooking or performing.
Tests were done involving leveling up dexterity to see how it progressed and the results are posted below.
Its damage goes up 1 die for every 15 points of dexterity and its modifier goes up by +1 every 4 points of dexterity.