Travel to Grom's Monument in the Demon Fall Canyon of Ashenvale and use Grom's Tribute at the base of the monument. Return to Javnir Nashak outside Orgrimmar before the Harvest Festival is over.
During the Harvest Festival of Azeroth, we give thanks to heroes for the sacrifices - in some cases ultimate sacrifices - they have given on our behalf. The Horde acknowledges many heroes, but none exemplify strength and sacrifice more than Grom Hellscream.
Help us honor Grom by taking this offering to his monument in the Demon Fall Canyon of Ashenvale. Place it at the base, and then return to me. I'll only be here through the end of the festival, so waste no time in giving Grom his tribute.
Grom's full name - Grommash - means "The Giant's Heart"; it took that and more to stand alone against Mannoroth. I think the Warchief said it best when the curse of the orcs began and ended with Grom. Fitting that Grom's sacrifice - and perhaps redemption - would be the wellspring from which the Horde flows today.
Thank you, <name>. You've paid tribute to a great hero today, honoring his spirit and the spirit of the Harvest Festival.