Sorry if this seems a bit obvious but I am 100% confused on how to do a power bite attack in vampire lord form.
The game says the way you level up the vampire perk tree is either killing people with the life drain spell or to kill them with a power bite but I have no idea how to perform said power bite.
I've tried using left and right triggers and all they do is take away some stamina and claw at the enemies.

It's a Power Attack kill. When they're low on health do a Power attack and you'll probably bite it, I've personally had more luck with the left bumper than the right.

i believe you need the perk first. when enemies are on low health and do a power attack with the claws it do a bite attack but this doesnt always happen for me

Ahh so it's like kill cam where if they're low enough sometimes it'll happen?

Yeah. But you don't need the perk to do it, like the person above you said. The perk only makes it so you regain health after the bite.

It can be controlled and it is NOT random at all. I can power attack bite whenever I want, it just takes practice and making sure someone (Like Serana...) doesn't kill the enemy before you have a chance. Basically, they have to be really low health and I'm pretty sure they have to be kneeling for this to work. Once they're kneeling PUT YOUR CROSSHAIRS OVER THEM and then do a power attack. I have successfully done this to multiple opponents in a row. To get them to low health you can poison them with your claws and let the bats svck their life away until they kneel.

Thank you both and at AsWeWept, I'll try this out, Serena has been with me a the whole time so I'll dump her somewhere for a bit!
*Edit* Just as AsWeWept said, worked perfectly, once again, thanks!