Formula to pull from Child Object to Master Object

I have a Master detail relationship from Sales Return Order Lines to Sales Retun Orders (Master).

I want to pull the description from any/all lines to a field on the Master.

What I have done so far is create a lookup to the Child, then create a formula field that pulls the description from whichever line i "lookup". But I need the description from mutiple lines pulled into this one/field on the Master.