The Warhammer of Zillyhoo is a weapon that appears in multiple adventures in both canonical and non-canonical contexts.
Problem Sleuth
The Warhammer of Zillyhoo is a non-canon item first seen in Problem Sleuth extra number 25. When Zombie Ace Dick transforms into Gummie Worm Zombie Ace Dick, Candy Corn Vampire Problem Sleuth becomes angered by ZAD's Truffle Shuffle-ing about. To defeat this foe, PS summons the Warhammer of Zillyhoo, a magical hammer with a name that is just as ridiculous as it looks.
"There she is. Forged in fire by the smiths of Pipplemop, commissioned by the sage Lord of the Wozzinjay Fiefdom in the Realm of the Snargly Fruzmigbubbins."
PS is overcome by the Warhammer's beauty, and he and ZAD collapse crying in each others arms because of how beautiful it is.
Homestuck
The Warhammer of Zillyhoo has also appeared in Homestuck under hammerkind. Gamzee appears to have the Warhammer of Zillyhoo in his jokerkind specibus and John has also obtained this weapon, after the bunny hugens it for his use.
The warhammer's beauty dazzled the Warweary Villein, the Courtyard Droll, and Karkat Vantas, John and Gamzee showing themselves to be the only ones who are not too overcome to wield it. The majesty of the hammer was such that John felt compelled to sing its name in a light, tenor gregorian chant towards a (imaginary) cathedral window.
Its cost is listed as "One Zillion", next to the symbol of a never explicitly named grist. As "Zillion" is only an idiomatic and not a real number, it is possible that the name of the unknown grist is Zillion and the costs is actually one unit of Zillion grist, though this contradicts the format alchemizing costs are usually listed in.
John had intended to use it against Jack, and show him its true might, but he was incapacitated before he could.
Some time after his resurrection, John is seen with a new hammer, named the Pop-a-matic Vrillyhoo Hammer by Hussie. Its alchemisation clearly involved the Warhammer of Zillyhoo and one or more Vriska related objects.