Hokay, some more formulated thoughts:
UR greaves: "-2 damage from energy type of choice (choose at start)."
Relic greaves: "-2 damage from all energy types." For this one I would also offer an alternate recipe, something like "can substitute X number of rare greaves in place of UR greaves (maybe like 10 or something- don't have to be unique)". Since this is a recipe that will be cycling, it could be designed more along the lines of sucking up some stacks of greaves/give people a reason to care about greaves beyond setting aside one or two (you'd have a better idea than us about how many greaves get mulched each year). Maybe if that doesn't feel relic-level enough it could instead be: "-2 damage from all energy types. Absorbs 1 melee or missile hit per game."
Legendary greaves: "-3 damage from all energy types. Can absorb 1 hit per game." Basically a strict upgrade from relic, with slightly higher resistance and the ability to absorb spells as well. Since this recipe is one that lasts forever, you could do something like "can substitute 8 different greaves in place of relic (presumably for acid, cold, shock, sonic, darkrift, fire, poison, sacred)", which wouldn't be possible until later down the line after the relic has cycled out.
Then change up the greaves printing slightly- start doing 2 elements each year, but the same total number of greaves (so instead of 5000 acid resist greaves, print 2500 each of acid and poison resist). Once all 8 types have been cycled through, start over again from the top, so greaves end up being on a 4-year cycle. Then, it lets people who are willing to stick around for a few years collect the pieces for a decent legendary if they do a bunch of adventures each year while simultaneously making greaves a more exciting draw from the treasure boxes for both newer players and older players who care about the secondary market.