I don't like the second option at all as it would make me feel like I had to save cheat. But if you want to go that route (spawning due to spellcasting), I'd prefer more forms of the crypt such as the following:
+1

Version after a spawn, unpillageable, in 40 turns ->
+1

+1

still unpillageable, in 60 turns ->
+2

+1

still unpillageable, this is the version that a worker builds, in 100 turns ->
+2

+2

Pillageable or castable
Or whatever the numbers are. I'd really like to be able to be rid of it if there's mythril or something underneath it. And I really don't want the bang-head-on-keyboard
50% chance of a bad result for something I'm going to be doing regularly (see also the discussion of the Master of Creation promo).
The other problem with this version is that you need an arcane unit to get your slave. Which means that in the early portion of the game which the mechanic is meant to address, you can't use it.