I'm pretty sure you do get an
Inv save, since it says that you don't get any
ARMOUR saves.
Inv save isn't an armour save.
But yeah Death has got it right. You get
D6 attacks (unless you roll a 1).
Also, you might want to look into going Mark of Khrone if you're going to get a daemon weapon. Allows you to roll an extra
D6 for attacks (sadly, the Roll to wound yourself works on both dices.)
Means on the charge you COULD get 17 attacks. 3 attacks for being a chaos lord, plus
2D6 (which COULD be 12), plus 1 for Mark of Khrone and then plus 1 for charging, and because the Daemon Weapon is a power weapon, you ignore armour saves.
Flipside of this though is, sadly, if you roll a one you lose ALL your attacks even the ones that are not to do with the weapon :(
Its awesome however, Played a game with that set up earlier. Took quite a few plague marines out with it and it was my main source of killing Plagues

Did roll 1 sadly on one of the turns.
Inv saved it so no big deal however