These 'invasion casks' would be part of the very rare random creature drop loot tables. A lesser cask would spawn 'X' amount creatures as an invasion. A greater cask would spawn 'X' creatures and continue spawning for 'Y' amount of time.
Break the cask to start an invasion at your location. Break the cask open inside justice and you catch Endangering the Public charges. Break the cask open in the presence of (or rooms surrounding) provincial leaders and you catch Crimes Against the State charges.
Different casks can be spawned, organized thematically:
A porcelain cask with a ghostly swirl - spawns random non-corporeals
A dusty cask caked in bonemeal - spawns random corporeal undead
A moss covered wooden cask - spawns random 'natural' creatures
A half-empty cask of rum - spawns random pirate mobs
A ratskin cask - spawns random rats, including plague/ichor rats
A shadowy pyramid of undetermined lightweight material - spawns PoP shadow creatures
etc.
The rare jackpot cask wouldn't spawn any particular type of creature, but when used in a hunting ground, it would spawn the engagement cap of mobs in every room in the hunting ground for 'Y' amount of time. Breaking a jackpot cask outside of a hunting ground would turn you into a glutinous lipopod for 'Y' amount of time or until you were uncursed by a cleric. For the duration of your lipopod status, when looked at; other people see a few of your random inventory items floating in your gelatinous form.
Microtrans Event, Quest and Festival casks could spawn creatures thematically sourced from those events.
Potency crystals could be used on lesser and greater casks to increase 'X', multiple times, up to X+N.
Infuser stones could be used on greater casks to increase 'Y', multiple times, up to Y+N.
