Great Rune achievement in Elden Ring

To do this, you must first beat your first demigod boss; Godrick The Grafted at the Stormveil Castle. Upon defeating him, you'll earn his great rune item, shown in your inventory as a Key Item.

Now, you must reach the Limgrave Tower Bridge; which is easily accessible from the Liftside Chamber Site of Grace. Once you reach the bridge, go across it, defeat (or skip) the 3 large enemies, and find a teleporter at the end of the bridge. Use it to get to the Divine Tower, and then climb the stairs to restore the rune and earn the achievement.

Don't forget to visit a Site of Grace to equip this Great Rune after restoring it. And don't forget to consume a Rune Arc to grant yourself the effects. This one grants +5 on all attributes. You must consume a new Rune Arc upon death to grant yourself the effect again.