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There's a few parts to the deception: Tricked to Death and Tampering with Food and Drink cover the means and method of the murder; it doubles down on the former if they mention something like "I didn't make them drink it, it was their decision". Beneath Notice covers weaponization of the fact that someone showing up to a party with free beer isn't going to face much scrutiny.
Can overlap with Beneath Suspicion if they evade scrutiny even after the fact (at least for a while) thanks to their disguise as "Bro in a polo shirt #14".
A variant of Nasty Party if they're using the gathering as a way to cross a whole group of people off their hit list at once- the mutation being that normally they're the one making the guest/victim list, but taking advantage of someone else's party with a convenient guest list works too.
Edited by Scorpion451
Though not specifically about the deception aspect, if the villain was snubbed and is poisoning everyone in revenge, then RevengeSVP is involved.
Hahaha "Bro in a polo shirt #14", but also thanks
The villain demonstrates this by showing up to a big party uninvited but nonetheless with free packages of beer. Unbeknowst to the party goers, the bottles of beer are filled with poison but the villain correctly deduces they'll accept the beer anyway without being suspicious because it's free beer and why would one be suspicious about a bundles of free beer.