" If you dominate this region put this card on yout power sheet…"
The problem (and the question) is that this card has a magic symbol.
If you remove the card from the board you are taking away the magic symbol also.
The domination is cheked before corruption tokens are placed, so this card will have no use for Tzeentch to advance the threat dial.
Why has it a magic symbol? only in case the Skaven does not have the supremacy?
It will happen a few times only…
I don't understand the purpose of the magic symbol in this card unless it remains on the board until the end of the curruption phase or something similar