Mythos of Illuna by paying its mana cost, targeting an Island you control.
Thousand-Year Storm triggers.
Holding priority, activate
Gogo by paying ({2} magic symbol) and tapping it, copying the
Thousand-Year Storm ability.
Resolve the copy of
Thousand-Year Storm's ability, copying the
Mythos of Illuna spell, targeting an Island you control that enters the battlefield untapped.
Resolve the copy of
Mythos of Illuna, creating a token copy of the targeted Island.
When the Island token enters,
Retreat to Coralhelm triggers, untapping
Gogo.
Activate the Island token by tapping it, adding ({U} magic symbol) .
High Tide triggers, adding ({U} magic symbol) .
Repeat from step 4.
Notes
If you've cast an instant or sorcery spell prior to starting this combo, you will create an additional copy of
Mythos of Illuna each iteration that can target any applicable permanent. You will also get a copy of
High Tide when you cast it, causing you to add an additional ({U} magic symbol) ({U} magic symbol) when tapping the Islands instead of ({U} magic symbol) .