Now I’m totally getting carried away. Could Grom Skullsmash actually be a dragonborn using Disguise Self? The limitations of the spell might help explain the hulking size unusual for a Githyanki.
That would make sense.
Felson Felgrim, the gambler who was murdered at Insight Park, had a note with Grom's name. Maybe Grom/the Dragonborn had made an appointment with Felson at Insight Park - and murdered Felson?
