Uses a one-handed weapon with nothing in the off-hand
When you adopt the Single Weapon Focus fighting style, you may not hold a weapon or shield your off-hand for the rest of the encounter.
You may swap your off hand / weapon hand freely so long as one hand remains the off-hand.
You may still throw Ichorwerk with your off-hand.
This hand still counts as a free hand for the purposes of spellcasting and other non-weapon abilities.