For each brother killed, you get 1 roll at the drop table.
Chance of a barrows item on each roll: 1 / (450 - 58 * brothers_killed)
All pieces at this point have even chance, but it is only chosen out of the brothers you have killed.
The piece chosen is remembered, and cannot be chosen again in subsequent rolls.
If you miss the chance for the barrows item, kills of creatures now matter. The total of the combat levels of the creatures you kill are tracked, up to a cap of 1,000. Brothers add to 656.
This number affects the chances of receiving each rune, AND the quantities. It is added to (2 * brothers_killed). Overall cap is 1012.
Code:
Reward: | Unlock | Req. for max quantity
----------------------------------------
Mind rune | 381 | 505
Chaos rune | 506 | 630
Death rune | 631 | 755
Blood rune | 756 | 880
Bolt rack | 881 | 1005
Half a key | 1006 | Always 1
Dragon med helm | 1012 | Always 1
50% extra runes for completing Hard Morytania diary.
Ultimately, for max rewards kill:
5 skeletons OR 7 bloodworms OR 5 giant crypt rats OR 8 crypt rats OR 5 giant crypt spiders OR 7 crypt spiders