Who do you think among the lords of hell, would most likely tempt a druid but also have the ability to supplement the druids abilities, so they can maintain their power level, even if in a twisted way?
Nine hells? What for and what they have to do with it? Now, say, Malar's minions are another business entirely...
People never wonder How the world goes round -Helloween And even I make no pretense Of having more than common sense -R.W.Wood It's not good, Eric. It's a gazebo. -Ed Whitchurch
Personally I feel the Wolf-Headed Amon might be an interesting option. He travels with a Winter Wolf, and has some ability (via fulfilling another's limited wish at will!).
My choice after him would actually be Glasya...who might do such a thing simply on a whim because she seems to be one of the more "chaotic" of Hell's Lawful Evil...and might be attracted to tempting a high charisma mortal Druid.
Baalzebul - in his guise as 'Lord of the Flies' would make an interesting choice for a corruption-druid (the circle of life through decay and entropy).
Geryon is known as 'the Wild Beast', so he also has a nature-vibe going.
Too bad the Hag Countess isn't around anymore (or is she?) - her connections to fey could be a good choice for an evil druid.
"I have never in my life learned anything from any man who agreed with me" --- Dudley Field Malone
Pretty much any devil or archdevil goes as any of them would do whatever is necessary to increase their power. I think Amon and Geryon are the prime suspects.
I have a dream that one day, all game worlds will exist as one.