If the target permanent is an illegal target when Dinrova Horrorâ€™s ability tries to resolve, the ability wonâ€™t resolve and none of its effects will happen. No player will discard a card.
If a player has no cards in their hand and Dinrova Horror returns a card to that playerâ€™s hand, the player must discard that card. They wonâ€™t have the opportunity to cast that card (or do anything else with it) before discarding it.
A token permanent returned to a playerâ€™s hand isnâ€™t a card and canâ€™t be discarded. It will cease to exist when state-based actions are performed after the ability finishes resolving.
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