question about death domain...

[Nemesis Revised]

will the death domain be any useful in terms of the summoning shadow avatar? I mean, will the avatar be similar to mummy dust that has no limit in leveling? Or will the shadow be limited to a certain level?

I had experimented with the summon shadow by casting my
wizzie's 'shades' spell (summon shadow). My shadow came out only as a level 16 (my wizzie is level 26).

The cleric death domain however seems to appear that it is like the summon of the blackguard. But do they have limits? I was reading documentation and saw that only mummy dust has no limit. Thus, will clerics of death domain be still any useful past level 20 if they know their shadow summon won't be any better further?