I personally think Barbatos/Scryb (or Dr. Viktor)/Growler is a really good deal.
Barb continues to be one of the more powerful monsters in the entire game. He can easily 1-shot an enemy monster, or potentially the whole enemy team with his 35dmg special-based AoE burning attack.
His extra turn skill basically means he's immune to possession too.
Scryb has a possession skill that grants him an extra turn. He has another single possession skill. So you can do TWO 95% accuracy possession skills on two enemy monsters. The usability of this is insane.
He can also give his team immunity to possession and to dark attacks.
Dr. Viktor is the 3rd fastest Undead monster. Only Hayman and Noctum are faster, but they won't be running 3 speed, so arguably Viktor will be the fastest. He has an AoE Stun skill AND an AoE Burn+Blind. In the Undead book, the only legendary immune to stun is Osteo. Amazing!
Growler is currently the only epic immune to possession. He's in the Undead book, as well as the Bad Legion book. He has a 2 turn possession! A single stun! And an AoE Debilitating Shadows (makes dark attacks 80% stronger).
I missed out on the Dr. Viktor deal <\3 Been wanting him forever... still don't have him!