This land enters tapped unless you control an Island or a Swamp.
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Drowned Catacomb is a workhorse dual land for any Dimir (UB) deck. Its conditional untapped clause is the key selling point: as long as you control a basic Island or Swamp, it comes in untapped, which in practice is most of the time after your first land drop. That makes it strictly better than a guildgate or tapland in nearly every two-color black-blue list.
The catch is the "basic" requirement — it cares about typed lands, not just any Island/Swamp. So in a deck packed with shocklands, triomes, and other typed duals it's nearly always untapped, but in a fetchland-heavy or basic-light build it can stumble early.
Play it in essentially any Dimir deck: control, mill, reanimator, theft, you name it. The only reason to skip it is if you're going hard on a fast/cEDH manabase where you'd rather run fetch-tutorable typed duals, or if you're mono-color. Otherwise, it's a cheap, reliable staple.
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 Island or Swamp, not for lands named Island or Swamp. The lands it checks for don't have to be basic lands. For example, if you control Blood Crypt (a nonbasic land with the land types Swamp and Mountain), Drowned Catacomb will enter untapped.