CONTAINS SPOILERS

Once you arrive at the location of Vegvisir The Elder, his runestone provides the hint "burn their young". This refers to the items you need to burn in the Ancient Bowl in order to summon him. You'll need Ancient Seeds. These are dropped randomly by the Greydwarf Brute or by destroying a Greydwarf Nest (see image). Burn 3 Ancient Seeds in the Ancient Bowl to fight The Elder.