Buy @ TCGplayer
Whenever a nontoken creature you control dies, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
Whenever you sacrifice a Clue, create a 1/1 white Human Soldier creature token.
If a nontoken creature dies at the same time as Ulvenwald Mysteries, the first ability triggers.
If you sacrifice a Clue as part of casting a spell or activating an ability, the last ability will resolve before that spell or ability.
The last ability is a triggered ability, not an activated ability. It doesn't allow you to sacrifice a Clue whenever you want; rather, you need some other way of sacrificing it, such as the activated ability that Clue tokens have.
