With Slab he has three different fights in the genocide and 4 in the pacifist. In the genocide he uses a sword he used back in the original monster/human war. He calls it Eon and it is a silver blade with a gold inscription that says

This is a Greek word and so will give you a shout out in my next art boom if you guess it. No looking it up though. He uses this sword to try to finish off frisk in the genocide and when he dies he says "Fine, you win. But do not take it lightly, you might need this and since you won, you get Eon." And he gives frisk the sword before dissolving. Also the gash on his arm is from the second battle where when you do finally hit him it is his arm and he teleports away thanks to Gaster.