The Ring of Spell Storing states (emphasis mine):
[…] While wearing this ring, you can cast any spell stored in it. The spell uses the slot level, spell save DC, spell attack bonus, and spellcasting ability of the original caster, but is otherwise treated as if you cast the spell. […]
So say somebody has acquired such a ring and casts counterspell or dispel magic using it, or is targeted by the Arcane Trickster Rogue's Spell Thief feature:
[…] If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. […]
[…] For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. […]
[…] Immediately after a creature casts a spell that targets you or includes you in its area of effect, you can use your reaction to force the creature to make a saving throw with its spellcasting ability modifier. […]
What would "your" or "its" spellcasting ability modifier actually be? Do you use the modifier of whoever put that spell into the ring? Does anything change if you have levels in zero, one, or two classes that can cast spells?
Some related questions:
- What is the spell attack bonus and spell save DC of a Thief using the Use Magic Device feature?
- What is the spellcasting ability of a Barbarian Totem Warrior?
- For the Ring of Shooting Stars, what is the spell save DC of Faerie Fire?
- What is the spell save DC for this magic item from Waterdeep: Dungeon of the Mad Mage?
- What is the Wand of Paralysis' Saving Throw?
- Do racial/feat/non-Spellcasting classes' spellcasting abilities count as a spellcasting ability for Staff of Power?
I believe my question is a bit different from these previous ones because the Ring of Spell Storing explicitly says that the spell uses the original caster's spellcasting ability and not the spellcasting ability of whatever creature is actually using the ring.
Best Answer
You use the spellcasting ability of the original spellcaster
You have quoted the relevant rules text from the Ring of Spell Storing (emphasis mine):
This is a clear cut case of specific vs general:
The general rules for casting a spell, as specified in the Casting a Spell section of the PHB Spellcasting chapter, are:
So, for the purposes of any spell cast using this magic item, the spellcasting ability (and consequently the spellcasting ability modifier) is changed from your normal spellcasting ability to that of the original caster.
What does this mean for the features you mentioned?
Casting Counterspell from the ring
Counterspell states:
But, for the purposes of Counterspell cast using a Ring of Spell Storing, the spellcasting ability is replaced from being your spellcasting ability, to that of the original caster. As a result, we use the spellcasting ability of the original caster to make this check.
Casting Dispel Magic from the ring
Dispel Magic is affected in precisely the same way, given the text of required check is the same (to a reasonable degree of accuracy):
Arcane Trickster's Spell Thief feature
This feature needs a bit more parsing, and as a result, I'm going to quote more of the feature's text than you have (emphasis mine):
I'm also, for ease of reference going to requote the relevant text from the Ring of Spell Storing (with a slightly different emphasis):
So, in this case, the casting of the spell has completed, and the Arcane Trickster is targeting you with it's ability, not the Ring of Spell Storing. So you need to make a check with your spellcasting ability.
This also has some implications for both you and the ring: