Fatal TzekHaar-Yt-HurKot are monsters spawned as part of the third challenge in the TzekHaar Front. Three of them spawn on wave 15, and will attack the player one at a time. Their attacks deal very high damage, and they are immune to all damage - the wave is completed by surviving four attacks from them, at which point they will automatically die. They attack with magic and ranged and can also use a black orb attack, which is a soft typeless attack that hits as hard as their normal attacks.
Taking any damage outside of Devoted hits will result in an failure of the challenge. The easiest method to deal with them is to use Barricade when they spawn, ideally lengthened by the Malletops perk and/or Turtling; otherwise, players may need to use Devotion and prayer switch according to the colour of the projectile, and using other abilities such as Resonance or Disruption Shield to deal with the black orb.