Question about cosmetic

I've seen the green flame explosion cosmetic but I can't find it at all in the shop to buy, anyone pls tell me

I think you might be looking for the Cinderswallow Urn Silver Flask.

@Eleven777#0993 I have used cinderswallow, and it's not it, it looks like this

Don't know if you found the answer somewhere else since its been more than a week but the screenshot shows the Automaton Herald effect specifically Herald of Ice.

If you look in the shop you can see it in the video at around 0:08.

Be aware that the other heralds look different so if your build doesn't have herald of ice you wont see this specific effect.