What are the mechanics behind those floating obelisks?

[Catts]

The ones inside rifts, they send you to limbo, do they just go after you ala creeper style? Does their teleportation go through blocks?

Also it seems now that you can randomly find yourself opening a door and ending up on the moon.

[Gorian]

I don't know anything about the creatures inside the dimensions, but it seems that the doors can lead to any dimension already on the server (the moon, the nether, mystcraft ages, etc.)

[jakalth]

The Obelisks are the guardians of the dimensions. They are there to try and make it so the player does not want to bypass the traps.

their mechanics work in that they can not move on their own. if they have line of sight, they will "charge up" their teleport attack, as in their eye opens. if they can not directly see the player, they will do nothing but wait. They do no damage to the player, only teleport them to a random location in Limbo. Even the obelisks in limbo will teleport the player to a random location in limbo if you stay in one place too long.

[Digdug83]

Don't forget that they creep the hell out of you. At least they do me.