Reach (This creature can block creatures with flying.)
Whenever this creature blocks a creature with flying, put a +1/+1 counter on this creature at end of combat.
If Frostweb Spider blocks a creature with flying that’s just powerful enough to kill it, it will die before it can get the +1/+1 counter.
If Frostweb Spider somehow blocks more than one creature with flying during the same combat, its ability will trigger multiple times.