The Bridge of Khazad-dûm

Creatures with power greater than the number
of cards in your hand can't attack.
"Suddenly Frodo saw before him a black chasm.
At the end of the hall, the floor vanished and fell to
an unknown depth. The outer door could only
be reached by a slender bridge of stone, without kerb
or rail, that spanned the chasm with one curving
spring of fifty feet ... They could only pass across it
single file."