The rifts are impossible-to-miss green and black rifts in the ground. As you approach a rift, it will spawn a ghastly specter or ghastly knight (scales, vv). Each rift is also guarded by a portal guardian (scales, even), which technically does not need to be killed but will aggro players trying to get close enough to the portal to close it.
Once the ghastly specter/knight has been killed, the rift becomes targetable. Stand close to it and use the Disintegration Vial.
Speak with Oomitelmora
Return to Qeynos Capitol District and speak with Lakosha Maera.