Current questions and answers regarding the map editor.

First, you can create multiple copies of the same creature and, depending on the hero's level, summon the desired one in battle. However, it's not possible to create a separate spell for summoning. If it's not critical, you can modify the original avatar – its file is located in GameMechanics/RPGStats/AvatarOfDeath.xdb.
Second, it doesn't work in the campaign.