It's funny because when Tirion destroyed it, it hurt him. XD He had like 28M health when he enters the Cathedral of Darkness, and the destruction of the heart took him to like 8M.

Fun fact: If you don't escape through the gate at the end of that quest, you will die. The combat log will say "The Lich King's Fury instantly kills (character)."

Fun fact #2: Ice Block totally absorbs it. It still says you were instantly killed, but you aren't.