sorry bad english

I am seeking a manga that I read some time ago and I forget the name

The setting is a fantasy world, I guess is isekai, I cannot remember correctly

But I can remember of just one arc that I never forget:

The protagonist is very powerful, he with his party will go search for a large dungeon, several different parties are searching the dungeon together, one party go first and is massacred by a male demon that looks like a mad scientist that can change his form, like metal liquid or whatever, this mad scientist demon massacre the adventures but is easily defeated by the protagonist and then this male demon became fall in love with the protagonist and change his appearance from to a female cute girl and keep together with the protagonist following his but he keeps all the time trying to keep her away...

someone know that name of this manga ?

Should be Nidome no Jinsei wo Isekai de.

~4400 manga completed
~1700 ongoing...

Yes, it is exactly it, thank you very much, Aremon.

Too bad I just find out now that the manga was cancelled. This is really sad, that manga was awesome...