Where do you get the weapon that fires super-heated lava/magma?

Hey first of all it's called the Magmaul and in adventure mode you will find the magmaul weapon the location of the magmaul is in council chamber (planet Alinos)after you defeat the lava monster there he will drop the magmaul.I hope that helped.

~skyline_2fast_luffy~

No offense, but you've asked this question twice. But you defeat the fire spawn you will unlock magmaul.

You get the magmaul in Alinos (obviously) on part 2. To get it, you must beat the Fire Spawn. An easy way to beat it is to just use the shock coil. Let yourself get hit by his fireballs. Because they're is a health expansion there. Getting the shock coil is a little more complicated. It is in Celestial Archives part 2. You have to go through this whole contraption of portals and enemies and you will eventually get it. Read other posts or a walkthrough how to get it because I'm not really good with explaining how to get the Shock Coil. Also, for future reference, on Arcterra part 2, you will get the Imperialist. Once again, you gotta fight an Arctic Spawn. Use the shock coil to beat it. Right before it throws it's ice balls, move to the side, while still draining it's health. I usually beat it before it even goes back underground. I am up to my 13th time beating the game. Eventually you will get very good at it. Take your time.

Hope this helped!
Koolstr