Talk:Mark of Syrix (5e Spell)
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Cast this on a creature (which apparently never misses and can't be ended with a save). Then, cast scorching ray as a 4th-level spell for five rays, then end the mark of syrix in the same turn. This 9th level character just did as much as 10d6 + 30d8 fire damage in two turns. A sorcerer could used quickened spell to do this in a single turn. Needless to say that's a little bit much. This only gets worse as you involve other party members' readied actions, and gets magnitudes more powerful as levels increase.
I don't think any spell with an effect of "when they take damage they take MORE damage" is really going to be balanced unless the "MORE damage" part is pretty minuscule or has a pretty strict limitation.