Lingering effects (poison, hold person etc.)

So this one is little more complicated:
You can set abilities to affect creature for a number of rounds (or minutes etc.). But the effect which changes over time would be useful for book-keeping:

Poison that damages ability of character every round on failed save.
Hold person which gives paralyzed status for creature till succesful saving throw

With the number to customize how many saves in a row are needed to stop the effect and customization if the effects are removed or only are stopped from getting worse.