Board Thread:Skyrim/@comment-11549959-20131229135241/@comment-218.186.252.127-20131229143043

The shout is counted similarly to a conjuration spell. So, you need enough space for a dragon to land and you have to shout at that space, which can't be too far away and Durnehviir is summoned in, like any other oblivion summon.

Having magic absorbtion can reduce the chances of summoning him equal to magic absorb chance, I think.