Could it be the Whomping Willow?
Ye may find me atop the eldest hill
- Dont think any books really specified a location?
I assist those in great need
- Lupin had great need of it during his years at hogwarts.
I shall take you in when I see fit
- The tree is fairly tempermental, although not mentioned, it might let some people in if its in a good mood?
For only then shall I concede
- Uh.
The "magic" I conceal from most
- "magic" could be a refrence to either the shreaking shack or Lupin way back when.
But ye must be prepared
- I doubt you would go up against the willow without some battle plan.
To brave the trials that I possess
- It'll hit you?
Really, really hard.
Those of faint heart, beware!
- Most people wouldn't dare go near the Willow, only brave people seem to make an attempt.
Just a gueessss. C: