This is a triggered mob, found in the Tegi cave on the north side of Umbral Plains.
To trigger him, kill the Netherbian Swarmcaller, which spawns in the same Tegi caves. The Swarmcaller should be alone in the center of a chamber which is reached by taking the right-hand fork from the second chamber, at approximately loc 2250,500. Note that there is no immediate effect from killing the Swarmcaller, but after one to two minutes, groups of four Netherbians at a time will begin to spawn in the same chamber where the swarmcaller was killed (making this sort of a mini ring event). The groups spawn about every five minutes, and there are always four Netherbians per group. The groups are usually made up of four Netherbian Ravagers, but there can be a Netherbian Flesheater, Netherbian Hive Fiend, Netherbian Infestor, or Netherbian Seeker in any given group.
When the sixth group of Netherbians spawns, the Swarmlord will spawn with them. Note that it is not necessary to kill the first five groups in order for the Swarmlord to spawn; if the chamber is left vacant after killing the swarmcaller, after 30 minutes, it will contain the Swarmlord and some combination of 24 other Netherbians.
None of the groups are difficult for a moderately-equipped 85+ character, nor is the final kill.