You are talking about the Cursed Cycle.

The cycle is composed of 3 mobs . The Glyphed Covered Serpent , The Exiled And finally The Cursed.

This event take place in the basement of sraa . To trigger the event you need to kill all the taskmasters in the basement and the 2 Rothz in a hour i think. You can know that the trial is up if the 2 Rothz are up.

Once you killed the 10 mobs , the Glyphed covered Serpent will spawn on top of the temple (in the basemenT) . Once you kill him the Exiled will spawn on the temple , and finally once he is killed the Cursed will spawn on the temple.

If you can kill MB , you should have np killing these mobs. You just need to joust the Cursed AE. The AE charm can be easily resisted with good resists.