The Poseidon offshore oil rig became one of the Enclave's mightiest fortresses, and eventually, the site of their greatest defeat.

Dragonskull Summit

This land enters tapped unless you control a Swamp or a Mountain.
{T}: Add {B} or {R}.

When the Planeswalker Angrath called dinosaurs "dragons," the name stuck in certain pirate circles.

Duskmourn: House of Horror Commander

Dragonskull Summit enters tapped unless you control a Swamp or a Mountain.
{T}: Add {B} or {R}.

Outlaws of Thunder Junction Commander

Dragonskull Summit enters the battlefield tapped unless you control a Swamp or a Mountain.
{T}: Add {B} or {R}.

Fallout

As this is entering, it checks for lands that are already on the battlefield. It won't see lands that are entering at the same time (due to Warp World, for example).

This checks for lands you control with the land type Swamp or Mountain, not for lands named Swamp or Mountain. The lands it checks for don't have to be basic lands. For example, if you control Stomping Ground (a nonbasic land with the land types Mountain and Forest), Dragonskull Summit will enter untapped.