Call of the old Ones

[Karol13]

Building spell of Fey, from the Orb, that gives XP bonus to newly produced units nearby, sometimes works differently.

It looks like the xp level does vary, based on some variables. The only variable I observed is non-Fey hero (when only 1/2 of XP is granted), compared to Fey hero (when full XP bonus is granted)

Which translates to different levels of produced units, like the dragons, having either level 6-7, versus level 12.

Is it really just the difference in the "native" hero, or other variables also play a role in this?

[Karol13]

Oh, it seems there is one more variable, and it is not the hero class.

Mirrorwood Trees needs to be near the Orb, and these trees needs to be stacked with Wisps.

If the spell was started before the tree was stacked, the spell must be cancelled and re-cast, in order to tap the full potential of the Call of the old Ones spell.

Also, there is a research, that increases spell level, which must impact the power of Call of the old Ones, too. (and here we can assume, that re-casting the spell is also required)