I would like to give my players the option of learning a way to make a spell or magical effect permanent. To this end I've created a homebrew spell, Spell Permanency (quoted below).
Before implementing this spell in game I would like to be aware of the potential ways in which this spell could be unbalancing and or exploitable. I have attempted to address some of the concerns raised by previous editions with limitations in the spell. These limitations may (or may not) be too severe.
Spell Permanency
5th level transmutation
Casting Time: 24 hours/level of spell being made permanent
Range: Touch
Components: V, S, M (diamond worth 1000gp per level of the spell being made permanent, one willingly given attunement slot of the caster or recipient of the permanency spell)
Duration: Permanent
While Spell Permanency is being cast the duration of the spell or magical effect being targeted is extended for the entire casting period. Once the casting has been completed the duration of the targeted spell or magical effect becomes permanent.
This spell can only make the duration of 3 spells, or magical effects, affecting the same creature permanent at any given time. For the avoidance of doubt, no creature can have more than three permanent spells or magical effects active on it at any given time. Any casting of this spell which attempts to make a spell, or magical effect, on a creature permanent beyond this limit fails.
During casting the caster must be in constant contact with the object or creature upon which the target is affecting. In the case of a portal being made permanent the caster must have one hand placed through the portal for the duration.
While casting this spell the caster is unable to gain the benefit of resting, and suffers all of the effects of lack of rest. While the spell is still being cast, levels of exhaustion gained from a lack of rest can be removed from the caster by Greater Restoration. After the casting has finished, these levels of exhaustion can be removed in the normal way.
Spells or magical effects that have been made permanent can have their permanency removed only by the use of the following spells: Wish, Dispel Magic cast at 9th level (dispel magic automatically fails if cast at a lower level), Greater Restoration (which only works if the creature who sacrificed the attunement slot is willing).
A spell or magical effect that has an alternate end condition will still end if that alternate end condition is met. For example a permanent duration polymorph will still end if the creature affected by it is reduced to 0 HP. Similarly a creature under the effect of a permanent invisibility spell will still end if the creature attacks or casts a spell.
If a spell or magical effect that was made permanent has it's permanency removed or ends prematurely the attunement slot used as one of the material components in the casting is regained.
Whether or not the casting of the spell is successful, the creature who gave up the attunement slot suffers three physical stresses which cannot be reduced or avoided in any way. These stresses remain in effect for 10d6 days. For each day the creature spends doing nothing but resting, eating and reading the total recovery time is reduced by 3 days.
- The creature immediately suffers one level of exhaustion, which is only removed at the end of the period of stress.
- The creature's Constitution drops to 3, if it isn't 3 or lower already.
- The creature's speed, for each form of movement, is reduced by 10ft. If this reduction would bring the creature's speed below 0 in any one form of movement, it instead drops to zero.
How balanced/exploitable is this homebrew spell?
Experience from other versions of the game where permanency was gamebreaking is welcome. Both in terms of how this homebrew potentially avoids those pitfalls, but also potential pitfalls it doesn't avoid.
Best Answer
Specific exploits
Simulacrum
With simulacrum, you have a perfectly loyal schmuck who can take all of those negatives for you. They can chill in their permanent Mordenkainen's magnificent mansion being waited on by 100 servants, while you run around with permanent invulnerability.
Wish
Wish can be used to cast spell permanency in 1 Action by duplicating its effects. This would allow you to make an unwilling target's spell permanent or to make a spell permanent on short notice. I cannot think of a specific exploit this would allow, but the behavior may be unintended. Wish will also waive the component cost, which should be minor anyways at the level you can cast wish.
This could be avoided by raising the level of spell permanency to 9th.
Amulet of Health and a warhorse
With some preparation, you can cast the spell yourself. The most debilitating of the three stresses is the reduction in your Constitution score. The rare magic item Amulet of Health sets your Constitution score to 19, which should override the reduction.
Now that you don't die from a stiff breeze, buy yourself a warhorse. You can cruise around at 120' per round as if nothing happened. Better yet, splurge on twelve warhorses - they tend to evaporate at the first sign of fireball.
Speaking of fireball...
Delayed blast fireball
Delayed blast fireball could theoretically deal infinite damage. It starts at 12d6 damage and increases by 1d6 every six seconds. After waiting one day, it will deal 14412d6 fire damage for an average of 50,442 fire damage on a failed save. I hope the target has Evasion or fire immunity!
If you can pick the field of battle, you turn said field - and anyone in it - into a smoking crater. If you can't, you'll need a way to move the bead to where you want. Delayed blast fireball provides the following option:
As BlueHairedMeerkat points out in the comments, the spell doesn't say you can move the bead, only touch and then throw it. The spell says you can throw it, so I am unsure what happens if you choose to touch the bead, pass the save, and then not throw it. Would telekinesis work? What if you're teleported while holding the bead? Your DM may or may not allow you to carry your nuke around.
If your DM allows you to transport the bead, a character can start by making a Dexterity saving throw against the caster's spell save DC. A 17th level rogue can have a +11 to Dexterity saving throws, which rises to +16 standing next to a paladin with 20 Charisma. Throw in the resistance cantrip and a Ring of Protection and they can carry the bead completely safely.
Specific limitations
Concentration
You cannot cast spell permanency on a spell for which you are maintaining concentration. This is because any spell with a casting time longer than 1 Action requires concentration while you are casting it, which will end any other spell you are concentrating on. (Since the duration becomes "permanent", not "Concentration, permanent", I am assuming that any concentration requirement is lifted once spell permanency is completed.)
This limitation is lifted by using wish to duplicate spell permanency.
Balance
The current incarnation is very overpowered, especially for a 5th-level spell. Even as a 9th-level spell, its power would rival wish.
As Ben Barden says in his answer, this spell is nearly impossible to balance. Either you have enough restrictions that no one will use it, or you don't, and everyone will. As new source material comes out, you would need to evaluate each new spell for further exploits. It may instead be easier to create a series of magic items or long-to-infinite duration versions of existing spells, which could each be balanced separately.