I'll weigh-in as one of the four players in this event:
It was both awesome and terrifying at the same time!
I play a rogue and never actually engaged with the Executioner, but it was clear from the start that we were severely outmatched and our elf barbarian (tree of life) paid the ultimate price. Not only this, but the very environment turned against us when the Shadows (vestiges of the Executioner's previous victims) rose from the ground.
We made a scary (albeit ultimately successful) fighting retreat through the sewers of the dead city, pursued by the shadows, because (luckily!!!) the Executioner was too large to slip through the drains.
Overall, it was a great session and all four players were hugely positive about it. The genuine fear of the terrible creature and the frantic retreat, with the dwarf fighter carrying our fallen comrade, was immensely memorable. For me, that makes the instant death possibility an asset to the game.