Dragon Heist / Dungeon of the Mad Mage

Elder runes are ancient symbols imbued with magical power. With years of study, any creature that can cast the Symbol spell can learn an elder rune and add it to the selection of symbols that can be created with the spell.

There is a lot of knowledge in the list held by Nalan.

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Elder Runes in Undermountain can have either a Bane or Boon effect.

Most times the effect is random (but sometimes it is specified). To determine the effect, roll a d20 - on a high roll, it's a Boon effect and on a low it's a Bane effect. Since this is a d20 roll, it can be affected by Inspiration and the like.