The Stone of Erech is the place where Aragorn must stand to call up the Dead to fulfill their pact to fight with an heir of Isildur against Sauron. Back in the days of the Last Alliance, the King of the Mountains swore on this Stone that he would help Isildur fight Sauron. But he was a squirrely little fellow, and he reneged on his agreement, choosing to hang out in the mountains instead. That's why Aragorn must summon the Dead at this precise stone. It's the site of his ancestor's oath with the King of the Mountains. This stone is black (death, anyone?) and also brought from Númenor by Isildur (symbolizing ancientness and Aragorn's origins).