We finally know what he was talking about!
Larian disclosed that the hidden content Newbon spoke of was in reference to portions of the Blood of Lathander quest line that rewards players with a Legendary mace.
“There’s a quest off the beaten path at the Rosymorn Monastery that rewards you with a nice mace called the Blood of Lathander,” Larian said. “If you’re not careful or you try to steal the mace, you may activate a terrifying solar weapon. If you (intentionally or unintentionally) kill Astarion with this solar weapon and then revive him once you’re back at camp, he… will have some choice words for you.”
Retrieving the Blood of Lathander feels more like a scene out of an Indiana Jones movie than a Dungeons & Dragons campaign. If you attempt to steal the weapon, a solar weapon begins charging and will blow up basically the entire zone, unless you can disarm it.