How can you create a combo with Goblin Welder, Retreat to Coralhelm, Seat of the Synod, Darksteel Citadel?
user
[START OF COMBO]This combo can be formed with Goblin Welder, Retreat to Coralhelm, Seat of the Synod, Darksteel Citadel

Color identity: UR
Mana cost: 
Steps:
  1. Activate Goblin Welder by tapping it, causing you to sacrifice Darksteel Citadel and return Seat of the Synod from your graveyard to the battlefield.
  2. Seat of the Synod enters the battlefield, triggering Retreat to Coralhelm, untapping Goblin Welder.
  3. Activate Seat of the Synod by tapping it, adding {U}.
  4. Activate Goblin Welder by tapping it, causing you to sacrifice Seat of the Synod and return Darksteel Citadel from your graveyard to the battlefield.
  5. Darksteel Citadel enters the battlefield, triggering Retreat to Coralhelm, untapping Goblin Welder.
  6. Activate Darksteel Citadel by tapping it, adding {C}.
  7. Repeat.[END OF COMBO STEPS]

Result:
  - Infinite blue mana
  - Infinite colorless mana
  - Infinite landfall triggers[END OF COMBO RESULT][END OF COMBO]

Other prerequisites:
  - Goblin Welder must be on the battlefield
  - Retreat to Coralhelm must be on the battlefield
  - Seat of the Synod must be in the graveyard
  - Darksteel Citadel must be on the battlefield
  - Goblin Welder does not have summoning sickness.
[END OF COMBO PREREQUISITES]
assistant