While this card can be a capable threat in any slower deck that values draw effects like this, you typically need a board of Merfolk for Svyelun to shine
If, for one of the targets, the player can't choose a new legal target, then it remains unchanged (even if the current target is illegal)
Besides being a great addition to any control deck , Thalia is also an easy-to-cast three mana with one color symbol, and she can take down a three-toughness-or-less creature without having to trade
Whenever she Quests, she can exert another character, but she can also stop characters from exerting, to prevent them from singing songs too
Its harsh, but I am of the opinion that, if you arent drafting, you arent really playing Magic
Not everyone can use a 10-drop that has to be hard cast after all