Use him as some kind of forest sage that provides the party with guidance to find the evil that is poisoning the forest and using animals in ritualistic sacrifice.
Then he ends up getting poisoned by the evil and becomes ambulatory, so the party has to kill him or cleanse him using the knowledge they've gained thus far.
... I'm not a DM, but I love coming up with stories.
Depending on how long it's been since you've played, they've changed the rules probably like between two and five times since then, so you'd have to learn how to play pretty much from scratch anyway!
Every time a new army book is released, they reset which models are in the game, and how those models are. Some armies get drastic changes which basically makes you buy, and paint, all new models. The rules regarding how painted your minis needed to be in order to participate in official Games Workshop events changed, but if you actually paint your models it's not really an issue.
Also, if you played a foot-heavy army in 40k, the 2" of 2 different model rules changed the way things moved significantly.
Warhammer (aka Fantasy) stopped existing, and was replaced by AoS in the 2010's. So if you played fantasy and not AoS, it would be a huge change.
AoS (Age of Sigmar) is still fantasy in genre, it replaced WHFB (Warhammer Fantasy Battle) which GW killed off. The rulesets (and available armies) are very different. The easiest change you can see even in pictures is the round bases, and no movement trays.
They may claim otherwise, but it was money.
I liked AoS more than WHFB, but it was definitely money.
Even if it wasn't to sell all new models (which they do with every new army book that comes out), they changed the names of generic races to things that they can trademark. Elf turned into "aelf", dwarves turned into "duardin", lizardmen turned into "seraphon", etc.