Okay, if we're talking antagonists-only, then yeah John Out West makes an excellent point that balancing against PC abilities is completely unnecessary.

So, the idea is that they trade hp for magic then? In that case, what I'd advise is that you forego spell slots - and by extension spell levels - entirely. If they cast from hit points, then they can keep casting so long as they have hit points.

Any unique magical abilities possessed by this cult should be wholly unique to them and unlearnable by the PCs (unless they actually join the cult, which should be a process that strips them of PC status).

Also, don't call them Warlocks, because that may lead to confusion with the two-spells-per-short-rest PC class. Instead, call them Blood-Mages or something else descriptive of their unique form of magic.


Some fun facts if you want them: There is an optional mechanic called Spell Points that can be used to replace the spell slot system of most spellcasting classes (Warlock is the big exception because of their unique spellcasting mechanics). In it, you use spell points to 'generate' spell slots of a certain level depending on how many points you spend. You can determine how many spell points a caster should have by converting their available spell slots at any given level to spell points (according to the following table) and adding them up.

Spell Level Point Cost
1st 2
2nd 3
3rd 5
4th 6
5th 7
6th 9
7th 10
8th 11
9th 13

Note: Yes, there are some rules to stop characters from spamming spells of 6th level or higher.

So, you might use this as a rough guide to hp costs per spell for your cult. Maybe multiply the point cost by the spell level to get an hp cost?

...actually, now that I think about it, it would be absolutely terrifying if, upon taking damage from the PCs, members of this cult got to use that damage to fuel a spell as a reaction.