While it was true that Rezen's modified ghost berries can corrupt Senior Beasts, it actually doesn't need him to become a Senior Mage.
As long as he has enough resources, he will be able to elevate the farm to the next rank, and it will have the ability to grow spirit plants at the Senior Rank.
"I hope that is indeed the case and you are not lying" the Guardian said with a serious tone. He was willing to take a risk and gamble on Rezen all in the premise that he really can corrupt Senior Beasts in the future.
If he were to find out that Rezen was lying, then...
The Guardian's eyes sharpened, and Rezen clearly saw that, and he hastily said "Rest assured that I am not lying. I can vouch for this with my life"
