Switching gameplans from beatdown to deterministic combo is another huge upgrade over the Rats decks
Plus, for a rundown of all the cards across the set and where you can find them, check out the Collecting Dominaria United article, which includes details on the Lost Legends cards
Both sides of Valki are excellent, with Valki being hand disruption useful in the early game to steal a powerful creature (with the potential to turn into it later), while Tibalt is a great end-game card, capable of removing multiple creatures and stealing cards from your opponents' libraries to cast yourself
However, in the right deck, where the graveyard can function like a second hand, it becomes one of the best draw spells in Magic
Id play this card if each of its abilities gave some other kind of bonus