"Druselsteinoween" is the second Halloween-themed episode of the Disney Channel animated series Phineas and Ferb. It premiered on October 4, 2013.
Plot
When Vanessa finds out that Doofenshmirtz has inherited a Drusselstein castle right next door to her, she takes the opportunity to hold the biggest ever Halloween party ever. Everybody in town are invited, including Phineas and Ferb, who act as party planners and DJs. Meanwhile, Doofenshmirtz gets Perry to help him look for a large, hidden treasure that his great aunt has hidden somewhere in the castle.
See the article on Druselsteinoween on Fandom's Phineasandferb wiki.