The orb in his chest doesn’t spin when he uses Archon (or Twilight Archon).
Edit: And his Dark Nexus skin’s Archon’s death animation uses his old/wrong Archon model/animation/effects, I can think of a solution: scale up the skin’s unique death animation so that it matches the larger model of Archon and uses it as Dark Nexus Tassadar’s Archon’s death animation.
(If you don’t know, this skin has a unique death animation instead of ragdoll physic.)