Blood of Life
If you are not undead, quaffing the Blood of Life will allow you to resurrect once, if and when you die; "Resurrect with the Blood of Life" will appear as an option on the death dialog. If you are undead at the time of quaffing, the Blood of Life will have no effect, and if you are undead at the time of death, you will not have the option to use it to resurrect. Turning into a lich after quaffing the Blood of Life will nullify it entirely (and give a message about your body rejecting it) even if you somehow become living again later, but becoming temporarily undead via other means (such as a belt of unlife) is fine, so long as you are not undead(via a belt or any other means) when you quaff and are not undead when you die.
Infinite Dungeon players: You cannot store multiple Blood of Life lives. Quaffing a second Blood of Life when you already have one in your system will give the usual message but have no effect. You must use up the first life before you can benefit from drinking another one.