Synth Eradicator

Haste
Whenever this creature attacks, exile the top card of your library. You may get {E}{E} (two energy counters). If you don't, you may play that card this turn.
{T}, Pay {E}{E}{E}: This creature deals 3 damage to any target.

Fallout

Haste
Whenever Synth Eradicator attacks, exile the top card of your library. You may get {E}{E} (two energy counters). If you don't, you may play that card this turn.
{T}, Pay {E}{E}{E}: Synth Eradicator deals 3 damage to any target.

Haste
Whenever Synth Eradicator attacks, exile the top card of your library. You may get {E}{E}. If you don't, you may play that card this turn.
{T}, Pay {E}{E}{E}: Synth Eradicator deals 3 damage to any target.

You pay all costs and follow all normal timing rules for the card played from exile with Synth Eradicator's second ability. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.