"All dead, all rotten. Elves and Men and Orcs. There was a great battle here long ago, yes."
—Gollum

Urborg, Tomb of Yawgmoth

Each land is a Swamp in addition to its other land types.

Pioneer Masters

Tales of Middle-earth Commander

"Yawgmoth's corpse is a wound in the universe. His foul blood seeps out, infecting the land with his final curse."
—Lord Windgrace

The List

If an effect such as that of Magus of the Moon causes Urborg to lose its abilities by setting it to a basic land type not in addition to its other types, it won't turn lands into Swamps, no matter in what order those effects started to apply.

Land cards not on the battlefield aren't Swamps while Urborg is on the battlefield.

Urborg's ability causes each land on the battlefield to have the land type Swamp. Any land that's a Swamp has the ability "{T}: Add {B}." Nothing else changes about those lands, including their names, other subtypes, other abilities, and whether they're legendary, basic, or snow.

Urborg, Tomb of Yawgmoth isn't a Swamp while it's not on the battlefield.