How would you handle corruption from an evil magic item?
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In general: By corrupting the player as well. That is, the item rewards something the player (not necessarily the character) really wants (or punishes with something the player wants to not suffer): XP, increased healing, increased damage… but only if they do something that is questionable, with the questionable thing being dependent on what behavior the item wants to achieve.
I originally planned for the control of werewolves power to only work when they are not in werewolf form but perhaps they can still influence other werewolves when they transform into a werewolf but in some unexpected way?
It could make them extremely charismatic to wolflike creatures, who will treat it with respect?
The player knew the ring was evil and would cause corruption if they used it. The chose to put it on when a actual werewolf was close by. Obviously hoping that they could control the werewolf. Perhaps they still can but they are corrupted somehow?
Wearer fails the corruption RR. Now what?
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The failed RR gave the item a certain degree of power over the character, and now it can punish him for not doing werewolfy things. They feel a certain longing for the moon, and not seeing it when it is up gives -1XP per second. Once he the PC has settled into just spending the night outside, the next step could be wandering along in the woods at night during a full moon (again -1XCP per second of not doing this when the moon is up). Once they get used to that as well and accept it, the next step follows, and another, etc. Eventually, they will have done a given number of steps on this escalation ladder (say, ten), and they become a full werewolf as per the usual rules.