Does it just restore 20 hp?
It does restore 20 hp, but if you take it and use one on the statues outside of relic castle. Which is found in the desert resort. ( If you Didn't know )
You can get a Zen Mode Darmanitan.
These Zen Mode Darmanitans turn into zen mode after they reach half health, but beware when trying to catch him, that he knows Flare Blitz and you are in a sandstorm, so its very easy for him to kill him self. ( Or Faint )
Hope This Helps :D