Newbie about to take the DM reins here. I've created a story for a one-shot but I can't decide on what creature should be the primary antagonist for it.

A quick summary of the one shot; I'll have 4-7 players at level 2 whose goal is to infiltrate a Manor during a masquerade ball and steal a valuable gemstone. Along the way they might fight guards or animated armours. What the players won't know is that the gemstone is cursed and when they try to steal it, it'll summon a creature but I can't decide which creature to use.

I've narrowed down my choices to a Flesh Golem, a Wraith, Helmed Horror or Invisible Stalker. I realise some of those creatures are going to be pretty tough for level 2 characters but my plan is for the creature to TPK the party and use the events of the one-shot as a precursor to the main campaign I'm going to run and they can explore the fallout of their one-shot with their own characters later if they want to.

To DMs more experienced than I, which of those 4 monsters would you pick?