Extremely easy to utilize, but don't tell the devs.
To become invincible, just die! As soon as you enter the death animation, as in Reid becoming a ragdoll, return to main menu. Menu speed shouldn't matter too much as the game will pause when entering a menu, even if you're dead. Then, load back in and you're health bar will be at 0 and flashing red. Your weapons will be invisible and enemies will swing at you but not hit you. However useful, there are many downsides to this glitch. First off, the vampire sense only lasts for about half a second and cannot be toggled/held down anymore. Although, by spamming the vampire sense button you can still see the hearts of enemies and the screen will be slightly blurred due to the visual effect of entering and exiting the sense so frequently. You cannot lock onto enemies which limits the amount of usable abilities like any ultimate. You will not be able to gain any blood but will be able to use blood abilities. Therefore I suggest dying with max blood and using the abilities wisely. Command grabs will kill you no matter what in this state so recognize the startup and dodge, or parry, them. I would dodge because parries are harder to hit when not locked on and if you miss the parry then you die instantly. if you do get command grabbed, be prepared to do the glitch again as you'll be entering a death state soon. Try not to hit a blocking enemy as you will be stuck in place for a moment and will have to dash to regain the ability to move. This glitch is fixed by either dying and failing to do the glitch or resting. I haven't done any extensive testing with this glitch yet but I will post anything else I find out about it. Hope this helps the runs! ...if anyone runs this game anymore feelsbadman
If you could find a way to make it where you could gain blood in that state then sure it could be useful. Or if anyone finds a way to have infinite blood then that with this glitch would be really good as basically god mode.