Toxic Ray

You shoot a purplish or greenish ray at a creature within range. Make a ranged spell attack against the target. On a hit, the target has the Poisoned condition and has 2d4+8 stacks of "slow toxin" in them. This spell has no effect on creatures immune to the Poisoned condition.

At the start of each of its turns, the target takes Poison damage equal to the amount of stacks of slow toxin in them. At the end of each of its turns, the target makes a Constitution saving throw, reducing the number of stacks of slow toxin in them by 1 on a failed save or by 2 on a successful one.

The Poisoned condition lasts until the amount of stacks of slow toxin in the target is reduced to 0, and the the amount of stacks of slow toxin in the target is reduced to 0 if the Poisoned condition ends early. In either case, the spell then ends for that target.

Stacks of slow toxin can't be added together. If a creature with stacks of slow toxin in them would be inflicted with more stacks, then the new stacks replace the old stacks if the amount of those new stacks exceeds that of the old stacks. The new stacks are lost otherwise.

Using a Higher-Level Spell Slot. The number of stacks of slow toxin inflicted on hit increases by 1 for each spell slot level above 3.