FINAL EDIT: @nomad@infosec.pub found it, it’s Roswell Conspiracies, thank you everyone!
The white-haired females would frequently meet at a Stonehenge-like monument where there was also some kind of a portal. I also only remember this one specific scene and nothing beyond that:
(a bit graphic, click to show)
There are men who are restrained, and their mouths are being held open. An elderly man places a large egg on top of a ramp, and the lower end of the ramp is connected to the mouth of one of the restrained men. The egg eventually hatches, and a big snail emerges from it. The snail then slowly slides down the ramp and enters the mouth of the man, who is forced to swallow it. As a result, the man is immediately brainwashed and gains additional physical powers and will serve as an additional soldier for that old man.
EDIT: the art style resembled that of X-Men Evolution
After looking deeper into it I don’t think this is right, though. Google is just making stuff up about shows that do exist, but every time I search into it it’s just wrong, lol
Yup, ChatGPT suggests that at first but then when you ask it whether it is sure it says actually no and apologizes because it has no answer. I checked W.I.T.C.H and it doesn’t have that scene, and it certainly doesn’t have those white-haired females.