History of Conqueror's Discipline

Conqueror's Discipline is a Triumph worth 10 score: complete a Grandmaster Nightfall strike with each subclass (Arc, Solar, Void, Stasis and Strand).

How to complete

1. Run a Grandmaster Nightfall once per element. The strike does not matter, and one class or several may be used. All five can be done in the same GM if the fireteam arrangement allows, since each completion only needs to be on the right subclass.
2. Locked Loadouts apply in Grandmaster Nightfalls, so you cannot swap subclass mid-run. Pick the subclass before launching and commit to it.
3. Suggested picks from guides: Well of Radiance Warlock or Sunbreaker Titan for Solar, Chaos Reach Warlock for Arc, a Sentinel bubble Titan for Void (called the best GM subclass), and a Stasis build that pairs freezing with damage resistance fragments.
4. Older guides (pre-Lightfall) list only four elements (Arc, Solar, Void, Stasis); the Destiny 2 wiki lists Strand as a fifth requirement. Check the in-game objectives.

Seal and rewards

Part of the Conqueror seal. Completing the whole seal unlocks the Conqueror title, which can be gilded, and per the wiki grants the Grandmaster Nightfall mission selection node. Gamerant describes this as the hardest triumph of the seal and notes it is earned permanently once obtained.