The fight is started by ringing the Cloister Bell on the north side of the roof, which triggers an awakening cutscene. After five kills the bell gains a Quick-start option to skip the cutscene. The fight has four phases: phases 1 and 3 are against Dawn, phases 2 and 4 are against Dusk.
Standard attacks. During the first three phases, Dusk uses a slash melee attack that hits up to 15, reduced by about 80% under Protect from Melee. Dawn uses a ranged attack that lands as two separate hits, each with a max of 9, and if you stand in melee range she may kick for up to 15.
Phase 1 (Dawn). Dusk is immune to all damage, so attack Dawn with Ranged, Magic or a halberd. Pray Protect from Missiles against her ranged attacks. She may launch a Stone Ball with a 3x3 area of effect that encases you in stone, acting as a 10-tick (6-second) stun and dealing up to 15 damage — move out of its shadow. Dawn flies up at around 55% health, starting phase 2.
Phase 2 (Dusk). Dusk becomes attackable and must be meleed (his armour blocks Magic and Ranged, and burn damage does nothing to him). After attacking once he charges a blinding attack, shown by his missing wing glowing — step one tile away to dodge it or take damage in the high 20s and get knocked back. Meanwhile Dawn rains a rockfall of 9-10 debris pieces with 3x3 AoE: edges hit up to 22, and a direct hit knocks you out, stunning you for 10 ticks. Never stand directly under Dusk, as he can trample you for up to 40 damage. Dusk reaching about 55% health begins phase 3.
Phase 3 (lightning, then Dawn). The pair charge a lightning attack that deals 5-10 damage per tick to anyone on or next to the vortices, one of which always targets your current tile — there is a one-second warning, so move to a clear spot. Afterward Dawn becomes attackable and launches three energy spheres that grow in charge stages; if she absorbs a fully-charged sphere it heals her 90 Hitpoints each, so absorb them yourself (which is penalty-free) unless your DPS is high enough to finish her first. Use the rock hammer once she drops below 10 health.
Phase 4 (empowered Dusk). Dusk absorbs Dawn's essence: his Attack rises to 300, his Strength, Magic and Ranged to 250, Defence to 150, and he gains one flat armour and grows to a 6x6 monster. He immediately uses a prison attack, dragging you somewhere in the arena and trapping you in a 3x3 cell of blue flames with one tile missing as the safe exit — fail to escape within 5 ticks and take up to 65 damage that also heals Dusk. He now uses both melee (up to 26) and a twin-hit ranged attack (up to 15 per orb, ~30 total). Prayer cuts his melee by about 80% and ranged by about 55%. The Wiki's simplest rule is to pray against the style of his previous attack, since his switches are otherwise hard to predict. Smash him with the hammer below 10 health to finish the kill.