That's why I tried the Frodo route, with a little twist. And Mount Doom wasn't meant literally - get Vulcan to hammer it, feed it to Tiamat, whatever. (means looking for an Oracle might be an idea)
More information will definitely be required, and casting ourselves as a villain would be a great way to get those hero-types to do all our digging for us. Of course, it won't stop them from killing us, but one problem at a time.
Unfortunately, until we know more about the item we have, we likely won't be able to say for sure what will and won't work. Does the corruption just make us evil, or does it also start making us not want to give it away at all?
Nobody ever kills a fair Riddle-Giver. The riddle-giver is occasionally hoist on his own petard, but we can avoid that by killing failures manually instead of depending on a bridge or other automated system. The riddle-giver occasionally gets greedy and tries to cheat the hero, and of course gets killed. But a legitimate, aboveboard riddle-giver? Safe. I very much like that solution.
If we're really good, we can construct our riddle to create the method of destruction for the evil artifact - being able to answer the riddle is not just a sign that the hero deserves it, but is actual constructive proof that the hero has the solution.
Problem is, we have a corruption artifact and stories like to be completed. By the time some farm-boy shows up with the answer, we might be so twisted we'll want to cheat to keep the thing...leading to our deaths.
I like the riddle idea, but I doubt we can make that into the solution to all our problems. It's just too easy.
Offer them a part from our hoard when they get it right; as soon as someone chooses our artifact tell them that only a true hero may get it and to prove that, fulfill tasks. Last task is to destroy the evil artifact after we explain the subterfuge ...
Oy! Whoa whoa whoa, what kind of dragon are we? GIVE AWAY our
HOARD? Sure, we've got to get rid of this thing, but at what cost? Some lines can't be crossed, even in the face of death!
Also that again risks the Evil Overlord wanting it, completing all but the last task, and then killing the 'virtuous guardian' to obtain it from our corpse. Lots of different stories around evil artifacts...
Oh good one there too. Heroes never go "nope" when presented with tasks to prove their worth in stories.
That being said, there are a fair number of scenes where villains approach the honest riddler/tester/guardian, are challenged honorably, and they ignore it and cruelly slay the guardian, looting everything.
What you said.
Also, if it's obviously evil, would a virtuous hero even select it as their desired reward?