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.