This might be an issue that only affects created sovereigns because some people have reported it working correctly, but it is definitely not working in my game. The shard seem to allow only a max amount of summonables, which is normal but they do not respawn when dead. So if I have 3 fire shards, I will get the 3 burning wraith creatures but when they die, no new burning wraith creatures will spawn, it becomes a one time thing. However, if I have 3 burning wraiths, save my game and then load up my game, 3 more burning wraiths will spawn. So I will have 6 burning wraiths even though my max is 3, this happens for all the summonables. Since I am using a custom sovereign and faction I do not know about the death shard summonable.