My Thoughts and Recherche
Since many people seem to struggle with this achievement and I think I quite well figured it out, I want to share my information to you guys.
In order to forcefully trigger this event I seached the community discussions about it and I found this post of gamer DatFancy:
Ive only gotten tempted once, and I believe it was at night and I was either corrupt, or close to corrupt, keep in mind this only happened once in my 180 hours, so its probably pretty freakin rare.
So I gave it a thought: If this statement of the devs is right:
Hey folks! I'm not going to reveal how exactly to trigger what event, but they are less random than most people assume.
Then it can't trigger, when you're at 5+ rot, nor can it happen with "any rot between 1 to 2" rot, since both cases happened many, many times in my games and I saw this event getting triggered twice now.
How to Trigger
So I guessed you need exactly 4 rot, so I tried it out: I got 4 rot, entered two forest tiles AT DAY (since there is this rumor this event only triggers at night, this myth is busted!) and it triggered on the second one. So I thought this might be really accidental - no. I could replicate this in the very next game in very similar fashion, but it took a little longer.
So in fact, this achievement is NOT very rare, but it's trigger is very specific. Since every player with 4 rot tries to rapidly get to 5 rot, cases where you wander around in forests with 4 rot are by itself just rare, and that makes this event triggering more rare than the other events. Plus, getting the idea that it needs exactly 4 rot makes the replication of triggering the event without knowledge almost impossible.
Of course, with every randomly triggering event you need a little luck. Don't give up hope, the event will eventually trigger.
I hope I could help you with this achievement and have fun with Armello!