How can I heal Talion?
From a nerdburglars question:
At the start of the game, you will be instructed to press circle or B to interrogate an enemy. If you do the same to a regular enemy that is not a worm, you will be able to draw the life force out of them and heal you. If you spot any rats you can do the same to them for a quick kill and some easy health. You can do this while in combat, but it is obviously difficult to do if other enemies are attacking you. The drain seems to go a bit slower when enemies are aware of your presence, so it pays to use stealth.