From Tar Valon Library
A Dreamspike is a ter'angreal that can prevent Traveling into or out of an area, limit movement within Tel'aran'rhiod and ward a person's dreams (ToM, Ch. 37). Each dreamspike has a key, allowing the people who know it to Travel within the area otherwise forbidden (ToM, Ch. 5, Ch. 38). It looks like a long spikelike piece of silvery metal, topped with a large golden head set with a golden inlay (ToM, Ch. 5). When activated, they are silver rods, two handspans high, made of fine threads of wirelike metal woven together like a braid (ToM, Ch. 35).
They can be placed in Tel'aran'rhiod, where a dome is seen, its size matching the area that Traveling is prohibited. Movement anywhere within this dome is possible, but a person cannot move directly out of it (ToM, Ch. 35). It is possible, however, to move through the barrier (ToM, Ch. 28), although doing so is not easy as it requires a high level of confidence in the Dream World. An inexperienced person, touching the barrier, may collapse, though it is possible for them to go in through it, for example, by using their momentum (ToM, Ch. 28).
Moridin locates two of them. One he gives to Graendal who uses Slayer to place it in Tel'aran'rhiod to try and trap Perrin. This one was destroyed dropping it into a nightmare of lava (ToM, Ch. 38). The other is likely to have been placed in the Black Tower, because neither can Grady (ToM, Ch. 47) or Rand enter (ToM, Ch. 51), nor can Pevara or Androl leave (ToM, Ch. 53, Ch. 56).