Twilight Sparkle drops a shard of crystal down a staircase to estimate its depth. Based on its fall time the object drops 490 meters, or just under 1/3 of a mile.
King Sombra is the first character in the series to be killed off.
Friendship Report: Twilight learned it's sometimes best to make a sacrifice for others rather than look after her own interests.
This is the first time in a two-parter that the main villain is stopped by someone other than the Mane Six. Spike is the one who delivers the Crystal Heart instead of Twilight, and Princess Cadance catches it and is the one to tell the crystal ponies to power it back up and defeat King Sombra.