i don't know if multiclassing for this feat is such a great deal as it might seem. You essentially get the benefits by paying two feats, 13 Cha, 13 Con, and the loss of any other multiclassing.
A fighter multiclassing into warlock just for Sacrifice to Caiphon can have tons of reliable powers, sure. But Cha is generally a pretty useless stat for fighters (Wis is slightly better as a tertiary) and not many of the warlock powers really mesh well with a fighter build. Fiendish Resilience? Unstoppable is strictly better.
Paladin/warlock is easier, but personally I think that's such a fun combo that I don't mind a little extra awesomeness
. The only other class that can really take good advantage of multiclassing into Warlock is the Bard. All other classes struggle at least a bit with MAD, since no other classes use Cha or Con as a primary stat.
On a separate note, if you want the hit point cost of the feat to always be roughly 1/5th of a character's hp for their max level power, 4 or 5 + level is pretty much spot on. Making it half-level will cause the scaling to favor Epic levels (which might be fine, since recovering powers is otherwise more common at Epic anyway).
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