Improvised Spellcasting (5e Variant Rule)
Improvised Spellcasting
When you take the cast a spell action, instead of casting a spell you know or have prepared, you can attempt to draw upon your knowledge and practice of magic to create a magical effect. This spell-like effect can be a duplicate of an existing spell, or can be an entirely new effect that you improvise. To do so, you must expend a spell slot of a level appropriate to the effect (no spell slot required for a cantrip-level effect) and make a spellcasting ability check.
The DM has great purview over what is and isn't possible with improvised spellcasting, taking into account what spells you already know and what spells are on your class list. For example, if you have spells like fire bolt and fireball prepared, they would likely rule that you could improv-cast with moderate difficulty an effect that duplicates or is similar to burning hands or scorching ray. Meanwhile, if you have no experience with necromancy spells and try to improv-cast a spell like ray of sickness, the DM might rule this as impossible, or at the very least give you disadvantage on the spellcasting ability check.
As part of casting an improvised spell, you must make an ability check using your spellcasting modifier, with the DC equal to 15 + the equivalent level of the spell effect. If you fail this spellcasting check, roll on the Magical Mishap table.
| d20 | Magical Mishap |
|---|---|
| 1-10 | The spell does not cast and nothing additional happens. |
| 11 | The spell does not cast, but 1d6 creatures of the DM's choice of a combined CR equal to or less than the spell's level appear in unoccupied spaces within 5 feet. These creatures disappear after 1 minute. |
| 12 | The spell appears not to cast, but its effects take place 1d4 minutes later. |
| 13 | The spell backfires. If it is a harmful spell, you are treated as the target. If it is a helpful spell, it is cast on a random hostile creature within 60 feet. |
| 14 | The spell does not cast and you take 2d6 force damage + 1d6 for each level of the spell. |
| 15 | The spell does not cast and a random magical effect determined entirely by the DM occurs. |
| 16 | The spell does not cast, but you destroy a section of the weave in your efforts to make it do so. The 5-foot space where you tried to cast this spell becomes as if affected by an antimagic field for 1 minute. |
| 17 | The spell casts normally but you gain a level of exhaustion from the exertion. |
| 18 | You cast the spell but it misfires. The DM chooses the target or decides it randomly. |
| 19 | The spell casts, but takes twice as long to do so. |
| 20 | You cast a random spell of equivalent level instead. |
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