This question is about the Rare magic item "Rod of Absorption" from D&D 5th Edition.
How the rod absorbs spells and gains levels of energy seems clear enough, my uncertainty is about how attuned spellcasters "convert" energy into spell slots. This section of the item's description is the relevant part (pg195 DMG, also in the SRD):
If you are a spellcaster holding the rod, you can convert energy
stored in it into spell slots to cast spells you have prepared or
know. You can create spell slots only of a level equal to or lower
than your own spell slots, up to a maximum of 5th level. You use the
stored levels in place of your slots, but otherwise cast the spell as
normal. For example, you can use 3 levels stored in the rod as a
3rd-level spell slot.
What exactly happens when you convert levels of energy into spell slots?
For example: consider Alice the variant human fighter 1* (no other multiclass levels) who has taken the Magic Initiate feat which includes the ability to cast a 1st level spell:
- she would be a spellcaster
- 1st level spell from Magic Initiate is a "spell known" (Page 3, Sage
Advice Compendium, Jeremy Crawford – published 2020 April 6th), - then attunes to a Rod of Absorption with at least 1 level of energy
Can Alice create a spell slot and use it to cast whatever 1st level spell she knows from her Magic Initiate feat even if she has already cast a spell using the Magic Initiate feat and by the rules of the feat "Once you cast it, you must finish a long rest before you can cast it again using this feat."
Is this an action under the Use An Object (PHB pg 193) rule that Alice can take at any time, even days before she would want to use this spell slot to cast her known 1st level spell?
Once Alice has created a spell slot for herself can she then end-attunement from the Rod of Absorption and keep this spell slot until she uses it?
If this is correct, can Alice spend 50 levels of energy from a Rod of Absorption to give herself 50 level-1 spell slots to cast the single 1st level spell she knows from Magic Initiate?
I'd expect that any spell slots disappear when used and wouldn't expect any spell slot created by Rod of Absorption to return after a long rest.
*PS: I know a level 1 fighter would not or should not get access to magic item as powerful as a Rod of Absorption, the low level is to focus the question on a PC with limited abilities.
Best Answer
You create slots AS you cast a spell.
The wording on the item is a bit confusing, but the following line makes it clear that the spell slots are not created to be used later:
The expression "in place of" is relevant here.
Since replacing the spell slots or creating them while casting, they cannot be kept for later (with or without attunement). Nor are they regenerated. Incidentally, creating regenerating spell slots would be completely unbalanced since the number that could be created would exceed the number of slots that even a high level caster gains. Even if slots of levels 6 to 9 are inaccessible.
The restriction on Magic Initiate is independent of spell slots.
Concerning Magic Initiate, the Rod only provides a spell slot for a spell and does not change any other restrictions. The restriction:
is independent from spell slots and therefore not affected by the Rod.
Magic Initiate makes you a spellcaster but it does not give spell slots.
The Rod can only provide spell slots of any level that you already have them. A fighter without Eldritch Knight martial archetype or multiclassed with a Spellcasting class does not have any spell slots so the Rod can create none in this case.