Actually instead of being the source of his power I was instead thinking that the souls within him would act as a way of keeping him from becoming an avatar of the Second ( yes I believe Malik ( or atleast the creatue that got him turned ) is a spawn of Second ).
The Eye did say Malik was : '' Completed, but kept whole by faith ''.
''Completed'' in this context likely means Malik is far enough evolved to become an avatar of his Apostle, but ''kept whole by faith'' could be that his consciousness and that of those who died for his sake are keeping him from ascending, like an anchor.
Hell maybe that's the reason they were sacrificed in the first place, to keep Malik, a level 5 perfect superhuman from becoming an avatar.
Ofc my theory has some holes, I mean the rituals for becoming an avatar need to consensual, but maybe the Second is powerful enough to force his spawn into submitting, so that's why Malik, Xanthe, or someone else performed a sacrifice on Malik, to keep that from happening, who knows