When Rageform enters the battlefield, it becomes an Aura with enchant creature. Manifest the top card of your library (To manifest a card, put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.) and attach Rageform to it.
Enchanted creature has double strike (This creature deals both first-strike and regular combat damage.).
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