If you're in a fireteam of two or three, Thalnok takes much less damage from the sword relic, making him near impossible to kill in one sword two downs (near impossible because people have done it).
BUT! If you're solo in your fireteam, and there happens to be people at the Court, regardless of whose rune is used, you will always do more damage to Thalnok with the sword, making it exceptionally easy to kill him in one sword two downs.
tl;dr thalnok takes less damage if you're in a fireteam and using the sword. He takes more if you're in a solo fireteam using the sword.
Note: The videos are not mine. Video credits to youtube user named someorangesherbert
3 Man Fireteam
As you can see, the amount of hits needed in a fire team and solo are very different. The one that solo-ed Thalnok even missed two melee swings in the uppercut combos in the video but still beat Thalnok in one sword.
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