Monster Spawners are technical blocks that are found in Vanilla Minecraft. They permit mobs to spawn in conditions they would normally not spawn in and to create streams of mobs to combat, allowing certain areas to be more difficult. The Better Dungeons mod adds a new type of Monster Spawner that is a one-time spawner, instead of having multiple of them, which permits the dungeons to be a bit more dynamic when they are created.
In Hexxit, Vanilla Spawners also generate in Battle Towers and any Twilight Forest structures. You can find Zombie, Skeleton, Spider, Cave Spider, Blaze, Silverfish and Creeper spawners naturally. The Not Enough Items menu contains many other kinds of spawners, including ones for custom mobs.
There are blocks that acts like vanilla Monster Spawners called Soul Cages. These consist of 5 tiers. Spawners can be used to fill 200 souls ("use" it to spawn, default right-click) in Soul Shards, which is used for Soul Cages.
In order to create a XP-farm in Hexxit, one can use a storage capsule station in order to displace the Monster Spawner.