Since Commander Masters is an all-reprint set with no specific plane or worldbuilding in mind, these commander precons pull from popular archetypes and settings across all Magic
You can't return the creature you sacrifice because it will still be on the battlefield at the time targets are chosen
Not every edition of the Base Set is this valuable, but a number of factors influence auction listings like the one making headlines now
Weve seen Cascade commanders before, like Yidris, Maelstrom WielderYidris, Maelstrom Wielder, Abaddon the DespoilerAbaddon the Despoiler, and my personal favorite, Imoti, Celebrant of BountyImoti, Celebrant of Bounty, but never one with such a steep requirement for the Cascade trigger
Void Stalker Its an Elemental, not a Spirit, but the two creature types are very similar in terms of it looks like whatever the art director and artist feel like it should look like. And for Void Stalker, its clear that they felt it should look like an octopus