Whenever one or more creatures you control deal combat damage to a player, investigate. (Create a Clue token. It's an artifact with ", Sacrifice this token: Draw a card.")
, Exile a creature card from your graveyard: Investigate. You gain 2 life.
(08/04/2016)
If an effect refers to a Clue, it means any Clue artifact, not just a Clue artifact token. For example, you can sacrifice Wrench to pay for Alquist Proft, Master Sleuth's activated ability.
(08/04/2016)
In a Two-Headed Giant game, if you control more than one creature that can attack, you may have them attack different opponents so that Ongoing Investigation's first ability triggers twice.
(08/04/2016)
You can't sacrifice a Clue to pay multiple costs. For example, you can't sacrifice a Clue token to activate its own ability and also to activate Alquist Proft, Master Sleuth's ability.
(08/04/2016)
Clue is an artifact type. Even though it appears on some cards with other permanent types, it's never a creature type, a land type, or anything but an artifact type.
