Player 1 on 8 points attacks with Darius Trifarian and kills Rift Herald at Aspirants Climb. Player 2 uses The Rift Herald Deathknell to summon Harnessed Dragon and targets the Darius. Does Player 1 still win for conquering or does the Harnessed Dragon resolve first?

To determine whether Player 1 wins, we must look at the combat showdown timing and how Deathknell interacts with combat resolution.

The Short Answer

Player 1 does not win immediately. The Harnessed Dragon played via Deathknell resolves while the combat is still active, which prevents Player 1 from conquering the battlefield at this moment.

Step-by-Step Breakdown

1. Combat Damage Step: Darius, Trifarian deals damage to Rift Herald, killing it.
2. Deathknell Trigger: The Rift Herald's Deathknell triggers and is placed on the chain. Because this occurs during the combat showdown, the game is in a "Closed State" (Rule 808.1.d.2).
3. Harnessed Dragon enters play: The Deathknell resolves, and Player 2 plays Harnessed Dragon. The Dragon's "When you play me" ability is placed on the chain, targeting Darius.
4. Harnessed Dragon resolution: The Dragon's ability resolves, killing Darius.
5. Combat Continues: Even though Darius is dead, the combat showdown has not yet finalized. Per RiftJudge FAQ #11200, because the Harnessed Dragon entered during the active combat, it is now a participant in that combat.
6. Establishing Control: According to Rule 348.2.a and Rule 461.3.d.1, control of the battlefield—and thus the ability to "Conquer"—is not established until the showdown fully closes (when both players pass priority in an Open State and combat resolution is finalized). Since the Harnessed Dragon is now present at the battlefield, the showdown does not end with only Player 1's units remaining.

Conclusion

Because the Harnessed Dragon enters the battlefield while the combat is still ongoing, it prevents Player 1 from being the only player with units remaining at the end of the showdown. Therefore, Player 1 does not fulfill the conditions to Conquer the battlefield at this time, and the game will proceed to the next steps of combat with the Dragon present.

This interpretation is based on the interaction between Deathknell triggers, combat showdown states, and the rules for Establishing Control (Rule 348) and Conquering (Rule 464). No official FAQ exists for this specific combination of interactions.