Lalli is a troll cook that guards the Golden Apple Tree. Players must speak with him during The Fremennik Trials to get the golden wool required to make the lyre, and then give him the recipe for stone soup using a pet rock. During his interactions with the player, he will be paranoid that the player wants to steal his golden apples.
Most trolls are named after the first thing they eat; if the first thing eaten is unknown, they're named after the first sound they make, which in this case was "lalli."
Stock
After the quest, the golden fleece can be bought for 1000 coins each from him.
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Trivia
In the original version of The Fremennik Trials featured in the RuneScape 2 beta, he appeared as the character Lalli.