Vitaath is part of the Helliquary. So to find him, you will first have to reach the end of the game, you will also need a level 41 character and have completed the main quest line.
To find it you will have to go to the Frozen Tundra, to the Bitter Hearth waypoint. The latter is found in the western part of the Frozen Tundra at Everfrost Lake. The boss will be near the waypoint.
Make sure to take out his minions before dealing with the boss, so you can deal with him more comfortably later. If you can take seven allies with you, do so, as it could be difficult to finish him off.
In this way we come to the end of this guide on where to find Vitaath in Diablo Immortal, so now that we are done, we hope we have been as helpful as possible and we hope that you will quickly find this raid boss.