I'm trying to work out a way to debuff an opponent in order to increase the chance of a spell succeeding against it. First of all, let me describe the situation:
The opponent in question is a Rakshasa-turned-Lich, with (AFAIK) the resistances and powers of both, so it’s not an easy one to defeat. What's more, this fiend-lich has created multiple phylacteries (in a horcrux-y sort of way) so fully destroying this foe is probably not a realistic option and the best I can hope for is an indefinite imprisonment somehow. For context, the PC is a high-level Fiend/Blade Warlock who should have access to a 9th level Mystic Arcanum by the time the confrontation takes place. Already-taken Arcanums are Soul Cage, Plane Shift and Demiplane. (The choice of what that 9th level Arcanum will be is currently unresolved — possible choices include True Polymorph or Gate).
Current ideas revolve around something like one of the following:
- True Polymorph (creature-to-object) followed by sealing that object up in a demiplane and throwing away the key;
- Gate spell to create a portal to Carceri, push the enemy through, close the Gate.
Option 1 would seem to be the more permanent of the two, but of course True Polymorph grants a saving throw, which the enemy would in all likelihood succeed in (+9 to WIS saves, with advantage, and that's before we consider the Legendary Resistances of a Lich, whereby it can simply choose to succeed at a saving throw 3x per day).
Option 2 doesn't involve any saving throw, as such, so that's good — but any magical means of pushing the creature through the portal tend to be low(ish) level spells or modification of low level spells. For example: Bigby's Hand, or Repelling Blast added onto Eldritch Blast. (Physically forcing it through the portal might work, but comes with a serious risk of ending up on the wrong side of the portal along with the enemy, so it's not ideal).
We should assume that this will be a 1-on-1 conflict taking place on the Prime Material Plane (though it could also potentially take place on a Demiplane created by the spell of that name).
That's the context. My question, then, is this:
I've been scouring spell lists (PHB, XGtE, TCoE, MToF) for possible high-level debuffs (since any spells of less than 7th level are a non-starter) that might reduce this enemy’s near-impenetrable magic resistance and allow a high-level spell such as (for example) True Polymorph to have a better chance of taking effect, but I've failed to find anything.
Can somebody point to something that I might have missed, either RAW or from pre-existing lore?
(Asking as a player)