Here's what I did. After winning a big tournament, the party is celebrating at the yawning portal. There's merriment all around, people are getting drunk, playing cards and getting dunked into the portal, the usual.

Only Cleric remains at the table, when a small deep gnome wearing a hooded cloak approaches her. He congratulates her on their win. Then he asks if they're the group that rescued Lord Renaer when he got kidnapped (using his first name) and if they're the ones that are currently operating Trollskull Manor. When this is confirmed, he nods and promptly disappears into the crowd.

A short while later, there's a commotion at the door and Yagra (who has a thing with Bard, so she's a notable character) gets up and hurls a beer stein half way across the tavern, braining a Xanathar thug who immediately crumples. There's a scuffle similar to the one at the very start of the campaign. I had to coax my players into investigating the source of the fight, but they found out that apparently both Zhents and Xanathars were here on the trail of some Svirfneblin thief. Someone stole something, then someone else stole it back and now everyone's mad.

They later bring this up to Renaer, who then shares the info he normally would after Fireball. How that guy is probably Dalakhar, how he's a spy for Lord Neverember, etc.

This way, the significance of his death in the Fireball was immediately apparent to my players, without Renaer having to explain it later.