Six brothers, six special effects, and the exact chest rates by kill count — from 1 in 11 up to guaranteed.
Barrows is the most repeatable loot run on Ashmarch. Six brothers lie in six crypts in Morytania; kill them, then loot the chest in the tunnels underneath. Every brother you have killed on a run raises your chance of a Barrows item from that chest.
The six brothers
Brother
Combat
Style
Special effect
Ahrim the Blighted
98
Magic (accurate)
Randomly lowers Strength
Dharok the Wretched
115
Melee
Deals more damage the lower his hitpoints
Guthan the Infested
115
Melee
Heals himself from damage dealt
Karil the Tainted
98
Ranged
Randomly lowers Agility
Torag the Corrupted
115
Melee
Randomly drains run energy
Verac the Defiled
115
Melee
Attacks ignore your prayer and armour
Chest reward chances by kill count — 6 to 24 kills: 1 in 11. 25 to 49: 1 in 9. 50 to 99: 1 in 6. 100 to 149: 1 in 3. 150 and above: guaranteed.
Dharok is the dangerous one and the reason you should not leave him for last on low health — his damage scales as his own hitpoints fall, so the end of his fight is far more dangerous than the start. Verac ignores both prayer and armour, so there is no defensive setup that makes him safe; he is simply damage you have to out-heal.
The chest also always pays out two rune rolls of 100–250 each, plus a potion. Ahrim's staff, the Dharok set and Verac's flail are typically the most valuable pieces.
Drop table
Taken straight from Ashmarch's own drop data. Rates are per kill on Regular mode with no drop-rate bonus.