Suggest me a name/label for A and B ;,>

a) a humanoid monster that eats human flesh, has superhuman strength & speed, And can form contracts/pacts with humans.
Weakness: silver and fire

b) the humans who formed contracts/pacts with
Weakness : Pretty much the same as

Tenkyu in advance :>

It sounds like a "demonic pact" ,but since many series differentiate between demons,daemons ,spirits... and give each their unique characteristics to these 'entities' and 'pacts'.

I would go for "supernatural pacts" or "pacts with supernatural entities" or something similar.
😕

For A this may nor be so original but 'Wendigo'.
For B maybe 'contract master'😕??