So I had a question when building a Mummy Lord, using the Undead Lord template from ToH:C. Mummy starts with DR 5/-. Undead Lord of 8HD gains DR10/magic, stacking with other damage reductions... how does one stack, DR5/- with DR 10/magic? DR 15/-? DR 15/magic? Other?
writing it out, you put DR 10/magic, DR5/-.
What it means in theory is that against non-magical weapon attacks the Mummy Lord has DR 10, against magical weapon attacks the Mummy Lord has DR 5.
In Rappan Athuk things got even more confusing in a few places.