Grimrock

Question:

Is there an easy way to use the swamp tileset underground ? without the need of a sky ?

Idon't understand the question.

Sure you can use the swamp-tileset without a sky-object - the result will be an overground-swamp with a black sky (even at daytime).

Or do you mean if you can use it underground? In an dungeon-area or thimething like that. Yes, that's possible, too. But your dungeon will have no ceiling at the places with the swamp-tiles. If you want a ceiling you have to define you own tileset with a swamp-ground an d anothe ceiling.