Visible "immortal" label with a running countdown marking a combatant who cannot presently be killed.
Scanning the frozen room in the Temple of Khepri, Carl noted that Princess Vinata was the only naga carrying the "immortal" tag over her head. Its countdown was frozen along with everything else, stuck at 63 seconds.
Carl did the math immediately: Vinata's immortality would run out 27 seconds before his own, giving him a narrow window in which she could be killed and he could not. The other five naga carried no such tag, relying instead on a full line of protections including aura protection.
A floating "immortal" label above a combatant's head with a live countdown timer beneath it.
Timed invulnerability whose expiry defines the kill window in the temple battle.