Nargott
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Yeah, that's a decent option.
Isn't the starting point too weak at levels below ten? Maybe give a starting bonus of at least 5 units? And if the hero rarely reaches level 20, then 25 initiative would be just right in that case.
25 initiative for undead, when Fortress starts with 16? That's too much.
The start is definitely not weak; starting from level 3, the avatar is already effective, and it costs the same as a level 1 spell (3 mana). An initiative of 3 means that the summoned avatar will get a turn in 3 rounds, which is quite good, considering that it's a high-level creature.
If it were possible to set a starting price, I wouldn't even use initiative, only stats. Initiative is a necessary element in balancing the price.