How difficult was the encounter as calculated, if I might ask? Deadly?
5 x 19th-level PCs: Storm Cleric, Four Elements Monk, Devotion Paladin, Champion Fighter Archer, Arcane Trickster.
CR25 Lich (all wizard spells known, 473hp)
2 x CR18 16th level undead spellcasters (same spell list as a Lich),
2 x CR10 spectral Githyanki Knights
2 x CR17 Adult Red Dragons without wings (possibly without Legendary Actions?)
The Lich-Queen Encounter
Total XP: 162,800
Adjusted XP: 407,000
Encounter XP Thresholds for this party
Easy: 12,000 exp
Medium: 24,500 exp
Hard: 36,500 exp
Deadly: 54,500 exp
Adventuring Day XP for this party
30,000 per 19th level PC
30,000 * 5 = 150,000
The technical term would be "Very Deadly" or possibly "Metal."