While Volos credibility may be questioned in the Forgotten Realms, theres no question that Volo, Guide to Monsters knows a thing or two about powerful creatures
Tyranid Swarm - Universes Beyond: Warhammer 40K Ive had games that have been won because my Haruspex has gained so much power that it deals huge damage to opponents, or converted its power to mana for a big late-game push
The +2 is basically Opportunity every turn, it picks off a creature or artifact if need be, and better yet, you can play whatever card goes away with the -3
It's great in decks with its size of 3/2 for 3 mana and in a pair of creature types to build around
But the best kind of Sefris enabler is a very on-flavor one for adventurers: so-called looting effects
Bolass Citadel can enable some broken stuff in any mono-black deck, especially one focused around artifacts