Blood from Every Dagger

(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.)
Choose two target creatures you control. Put a -1/-1 counter on one of those creatures and a +1/+1 counter on another one of those creatures. Repeat this process 13 times.
"They looked down, not able to muster enough courage to look each other in the eyes. The first drop hit Florina's hand. No turning back..."