Zhangar |
If Kostchtchie successfully destroys the Torc (probably through a minion getting lucky with disjunction, since he has to seize Baba Yaga's hut and boil the torc in her cauldron for 24 hours otherwise - and Dear Grandmother can whup his ass), then it would make sense to increase his CR and give him some additional abilities. (Like anything the torc does that he can't already do on his own.)
After all, he became a demon lord - a creature of spiritual evil made flesh - while missing part of his soul.
Getting that back would almost certainly make him stronger.
Also, keep in mind that if your PCs aren't mythic, then even the CR 26 Kostchtchie has the ability to insta-gib the entire party with mythic Blasphemy (effectively caster level 36 against non-mythic characters), even though the DC's a measly 25.
Of course, Kostchtchie's numbers will skyrocket if you bother to give him equipment - he's an extradimensional god-king, and should have some good stuff, after all.
(And none of that's even really considering his armies, because extradimensional god-king.)