So, as WoTC states, you will always get a rare or mythic rare in a regular booster pack, but if you find yourself in a mass market store and only have a couple of bucks to spend on scratching your Magic itch, you now have this as an option.
(2019-06-14) If the target player's library contains fewer cards than the sacrificed creature's power, all of those cards are put into that player's graveyard
Swanna 140/167 Loading
A full playset of a single evolutionary line, meaning four V and four VMAX/VSTAR that you could put into a deck, costs 7,100
Put Tendrils on top, and then Yawgmoths Will on top of that
The downside to not having a dragon handy is rather significant, whereas the upside is huge, possibly the biggest upside in the whole set for anyone who breaks them