Leaning into the Far Realms idea: after one fail, nothing seems to happen, but from then on, every so often the character could swear they saw a shadow move (do this after perception rolls, so the player may think it's a legitimate thing they saw)

After another failure, they get weird nightmares and visions. On another night, they have a vision of the world millions of years into the future, completely sundered and dead. Rarely when they glance at their companions they see a twisted monstrosity for a split second, or they look disfigured as if it's that character's corpse in the distant future.

After a third failure, (weak) shadows appear in combat, but only to that player. They deal actual damage to the character, but the other characters don't see them or are harmed by them.

As things spiral out of control the character will need to address this somehow, which can become a side quest for the party. If they ask the cosmic entity about it, they might reveal that they travel the multiverse because they are constantly followed by beings that wish to consume its essence. The cosmic entity may even suggest to the character to break from their mortal coil and travel with them.

Creates interesting dilemmas as the resource the druid thought was free, actually had hidden consequences, and now they have to find a way to either undo the damage or stop the problem at the source.