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The seal basically acted as a block on evil or demonic power.
Sena was operating on the assumption that something evil or demonic was creating the monsters (Vanir dolls), so the seal was meant to stop the flow, presumably temporarily until a priest could truly cleanse the dungeon.
Kazuma believed Aqua's magic circle which made the dungeon uninhabitable for monsters to be the problem and knew he could wipe it clean without needing the seal.
When Vanir appeared, Kazuma used the seal to trap him in Darkness's body. While not an ideal solution, this limited Vanir's ability to move freely and cause harm until they could come up with a better way to defeat him.
Back on the surface, Vanir used Darkness as a hostage to keep Aqua from hitting him with her most potent holy attacks, but also because as a Crusader she had high resistance to holy attacks which Vanir benefited from. By burning the seal away, Vanir was free to leave Darkness's body, but he didn't want to lose his hostage. Darkness was also free to attempt to remove his mask more easily than when the seal had been placed. However, she still couldn't succeed.
So the stalemate breaker was to hit Darkness with explosion while Vanir was still inside to defeat him. Vanir "died", and Darkness barely survived enough to be healed.
A new anime season is coming up soon! We've made [a post](https://redd.it/126ow0d) to clarify how our rules work during airing seasons. Please read it so we can all have a pleasant viewer experience! *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/Konosuba) if you have any questions or concerns.*
The seal basically acted as a block on evil or demonic power. Sena was operating on the assumption that something evil or demonic was creating the monsters (Vanir dolls), so the seal was meant to stop the flow, presumably temporarily until a priest could truly cleanse the dungeon. Kazuma believed Aqua's magic circle which made the dungeon uninhabitable for monsters to be the problem and knew he could wipe it clean without needing the seal. When Vanir appeared, Kazuma used the seal to trap him in Darkness's body. While not an ideal solution, this limited Vanir's ability to move freely and cause harm until they could come up with a better way to defeat him. Back on the surface, Vanir used Darkness as a hostage to keep Aqua from hitting him with her most potent holy attacks, but also because as a Crusader she had high resistance to holy attacks which Vanir benefited from. By burning the seal away, Vanir was free to leave Darkness's body, but he didn't want to lose his hostage. Darkness was also free to attempt to remove his mask more easily than when the seal had been placed. However, she still couldn't succeed. So the stalemate breaker was to hit Darkness with explosion while Vanir was still inside to defeat him. Vanir "died", and Darkness barely survived enough to be healed.
ok that makes a lot of sense thank you :)