The dwarves are celebrating the vengeance of many grudges, as only a dwarf can (such celebrations, if nothing else, tend to give human alcohol poisoning). While none of them are experts in grudgelore (and thus cannot keep account of exactly which grudges will be struck out as avenged), they have recorded events dutifully, and many of the myriad crimes of the greenskin race will be removed for such things. The Tsar also benefits; by performing such a campaign, he redeems himself of some of the crimes of his predecessors and lightens the burden of grudges upon the Kislevite peoples.
The beer is both of high quality and very strong - and if the Acolyte says this, he will be treated to the mournful tale of Joseph Bugman, the finest dwarf brewer to ever live, whose many Ale's, of which Bugman's XXXXXX was the finest, are renowned throughout the old world, and whose brewery was destroyed by perfidious greenskins. It's said he still lives, stalking the World's Edge Mountains as a ranger with his remaining followers, seeking to fullfill an oath to slay all those who killed his family and robbed the world of such fine ale. Stocks of those fine brews exist, and it is said a single sip of them can restore a dwarf's soul and bring health to the wounded, but they dwindle with every year that passes.
The dwarves will also tell him the broad strokes of history as the dwarves understand it. Dwarves were born in the ancient days of the world, as all races were, and the Ancestor-gods came from beneath the mountains to walk among them as mortals, and become their first kings and queens - Grungni their first king, the first to delve into the earth and return with it's riches, his brother, Grimnir, the first warrior and the first Slayer, their wife Valaya, the first mother, baker, brewer and healer. Gazul, her brother, who hunted monsters and tended the dead and Grugni's sons, Smednir, metalworker and smelter, Morgrim, the first engineer and the second king, and Thungni, the first runesmith. Under the rule of the Ancestor-Gods, the dwarves were prosperous and righteous, every just tradition being laid down in the image of these forefathers, but they departed one by one. The golden age that followed the principles of the ancestors was long and great, and came to an end when the treacherous elf-king betrayed the dwarves, leading to a war of vengeance that shattered both peoples forever. Now, many thousands of years later, the dwarves are a shadow of what they were. Their ancestors would be disappointed in them, but they will do their best nonetheless, even as they dwindle. The current high king, Thorgrim Grudgebearer, has declared this to be the Age of Vengeance, a final reckoning where the dwarves will see every grudge fulfilled in the dying age of their people, and spit in the eye of everyone who has every wronged their race or contributed to their current fallen state. Dwarf history is not a happy thing.