Multi class was confirmed on release additionally there is suppose to be mercs with live release so why restrict yourself to 1 class / race combination instead of a full party option. With speculation of level cap being 12 currently
Party 1
Half-Elf - 3 Hexblade / 9 Oathbreaker Paladin ...
Hexblade was published in Xanthar's Guide to Everything.
It is unlikely to be included in BG3.
The Hexblade subclass has already been modded in the game. Some modder, Kgboy, found parts of the subclass in the game, and finished it.
Its work in progress as different mods capture the features more accurately than others (In one area...then vice versa).
One is an item mod that gives a hexblade ring...this version implements Hex the best, although it renews on kill until 10 turns are up. Its once per long rest so this may be a good home brew (undecided).
Hexblade is my favorite class of all time...the ultimate Gish build.
