How to use Great Runes in Elden Ring

sportskeeda.com:

Great Runes are one of the many rewards that players will earn as they take down major bosses in Elden Ring. Each of these bosses is linked to the main story as a demi-god, so the items serve as a part of the plot and as a major source of power, along with Remembrance.

The best comparison that can be made for these runes is to Embers from Dark Souls 3, but with a lot more of a process behind the item.

Embers were a temporary boost to one's life, and Great Runes work the same way to an extent. But players have to go through a lot more work to get them.

Unlike the Embers items, Great Runes are incredibly limited and can be used indefinitely once equipped. These runes are dropped as rewards for taking down a demi-god, and when the full rune is attuned, players can take on some of their power for themselves.

But even if players earn a rune and equip it, they still need the necessary items for the power to activate.

Activating a Great Rune in Elden Ring:

Rune Arcs is what makes these runs similar to the idea of Embers. So, players will need to have them on hand if they want the boon.

Rune Arcs are consumable items that provide a boon when used. On their own, they can give a slight boost to HP and other small stats. When used in tandem with a boss rune, though, they are far more powerful.

With a rune equipped, one can consume a Rune Arc to use the power of the boss rune. Until players die, that power will remain and can be a massive boon in any battle.