I'm always interested in cool new ways to reward players, and whilst browsing through some 3.0 and 3.5 books, I came across a few old ideas that I really liked - namely the Planar Touchstone from the Planar Handbook, and the Artifact Spells from Secrets of Xen'Drik. What follows is a 5e spell based on these ideas, which I hope gives you some ideas about adopting something similar in your own games.
"In the middle of the wretched heat and mosquito haunted humidity, we came across an ancient Settari statue. Sardai was carved into it, which I of course innately understood, though it was pure gibberish to my Dundorin companions. Almost at once I realised that the runes spoke of a potent spell I could unleash, if only I could properly understand their message, and lock it within me. I set to work at once, and after a days' solid study, had mastered the magic!
What a shame the effort of casting it wiped it from my mind...and of course by then, we were in too much trouble to return to the statue so I could restore it! Ah well"
- Yardo Job, Explorer and Mage.
Touchstone Spell (Invocation)
To Learn: Intelligence (Arcana), D.C. 25; 24 Hours*
Casting Time: 1 Action
Casting Check: Arcana (Intelligence) D.C. 25**
Failure Penalty: No***
Area of Effect: 40' Diameter, 50' High vertical Cylinder
Saving Throw: Dexterity 1/2 and Partial
A column of searing acid erupts in the area, inflicting 77 (22d6) Acid damage to all creatures in the area and pushing them to the top of the cylinder. The acid then vanishes, and those creatures fall 50' with normal effects. A D.C. 20 Reflex save halves the damage and allows the target to avoid being lifted up.
Once cast, the spell is lost from memory, and the source must be studied for another 24 hours (Learn D.C. Intelligence (Arcana) 25.
* To properly lock the magic into your character's mind, you must study the spell for the total specified amount of time and then make an ability check (often skills will apply). You do not have to study the spell in one go, although you must do so in chunks of at least 1 hour. If you fail the check, you do not learn the spell, and must start again. Once you cast the spell, it is gone from your mind, and you must study it again as before. You can normally only hold one such spell in your mind at a time.
** Although you hold the magic, it is potent and difficult to bring forth. You must succeed in the specified check to cast the spell. If you fail the check, your action is wasted, although you retain the use of the spell.
*** Some spells have unpleasant effects if you fail the check. Usually, there is a safe margin for error, where nothing happens - so, for example, you may have to fail the check by 6 or more to trigger the penalty, or have to have failed to cast the spell twice in a row. The nature of the penalty is unique to each spell, and not all spells carry them.