Activate Scion of the Ur-Dragon by paying {2}.
Holding priority, activate Scion of the Ur-Dragon a second time by paying {2}.
Resolve the first Scion of the Ur-Dragon ability, searching your library for Moltensteel Dragon, putting it into your graveyard, causing Scion of the Ur-Dragon to become a copy of Moltensteel Dragon until end of turn, and shuffling your library.
Activate Scion of the Ur-Dragon by paying {R/P}, giving itself +1/+0 until end of turn.
Repeat step 4 five additional times.
Resolve the Scion of the Ur-Dragon ability from step 1, searching your library for Skithiryx, putting it into your graveyard, causing Scion of the Ur-Dragon to become a copy of Skithiryx until end of turn, and shuffling your library.
If Scion of the Ur-Dragon has summoning sickness, activate it by paying {B}, giving it haste until end of turn.
Deal combat damage with Scion of the Ur-Dragon, causing an opponent to get ten poison counters and lose the game due to lethal infect damage.

Target opponent loses the game

Scion of the Ur-Dragon

Skithiryx, the Blight Dragon

Moltensteel Dragon

Activate Moltensteel Dragon by paying 2 life.
Repeat step 1 until Greven has power 21 or greater due to its first ability.
Deal combat damage to an opponent using Greven. The opponent loses the game due to being dealt 21 or more combat damage from your commander.

Combos with "Moltensteel Dragon"