Inflict a magical contagion. Choose an ability. The target must succeed on a Constitution save, or else they take 11d8 Necrotic damage, are Poisoned for the duration, and have Disadvantage on all saves made with the chosen ability.
At the end of each turn, the target rerolls the save until it has 3 successes, ending the spell, or 3 failures, making the spell last for 6 days.
If anything tries to end the Poisoned condition on the target, it must succeed on a Constitution save or the Poisoned condition continues.
Quotation.
-Quote Source
Cast a spell of level 5 or lower that you can cast in an action targeting yourself. This becomes the contingent spell, expending its spell slot as well as Contingency's spell slot. The spell doesn't take effect until a chosen trigger occurs.
As soon as the trigger happens, the spell takes effect whether you want it to or not, and then Contingency ends.
You can only target yourself with the contingent spell, you can only have one contingent spell at a time, and the whole spell ends if you ever don't have the material component on your person.
See the Player's Handbook for the exact rules.
Spells casting spells seems like a bad idea. Let's try it.
-Abjurist Ovtuch Irieya moments before major arcane breakthrough