From Archon Arcana - The KeyForge Wiki
The active player may play cards of the active house during step 3 of their turn. When a card is played, the active player takes the appropriate card from their hand and does the following:
- Reveal the card being played and confirm it is eligible to be played.
- If the card is an action, continue to step 3. If the card is an artifact, a creature, or an upgrade, it enters play and is placed in the appropriate play area:
- If the card is a creature, it enters play exhausted on either flank of the controlling player’s battleline.
- If the card is an artifact, it enters play exhausted in a row below the controlling player’s battleline.
- If the card is an upgrade, it enters play attached to a creature chosen by the active player. An upgrade can be attached to a creature controlled by either player. The upgrade is controlled by the player who played it, even if the creature to which the upgrade is attached is controlled by the opponent.
- The card is considered “played” for the purposes of card effects. Resolve bonus icons (if any) from top to bottom, one at a time.
- Resolve “Play:” abilities, “after a card is played” abilities, and ”after a card enters play” abilities in the order of the active player’s choosing.
- If the played card is an action card, it is placed faceup on top of the owner’s discard pile.
—KeyForge Master Rulebook v17.1 September 2023