Dash {3}{R}{R} (You may cast this spell for its dash cost. If you do, it gains haste, and it’s returned from the battlefield to its owner’s hand at the beginning of the next end step.)

As long as Undead Battlerager has haste, it has "When Undead Battlerager becomes blocked by a creature, it deals 3 damage to that creature and 3 damage to its controller."

When Undead Battlerager leaves the battlefield, add X mana to your mana pool until end of turn, where X mana is the amount of damage it dealt this turn.