Predecessor Builds > GRIM.exe > Terminator 1.1 OHB

Some times I take Grim mid, he's a powerful character even built poorly late game, but there's certain items that hurt other carries that he can take that make him a monster.

Reasons to take Grim #1 Enemy team has a lot of healing, maybe a Ramp, Khai, Grux. A good way to set yourself off is to go straight into tainted rounds, ordinarily I avoid building these on anyone other than Murdock, there's just no reason to build them on characters who's position should be solely pure ADC. No one is out regenerating you providing you build right on a traditional carry, (attack speed, power, %pen) but in Grims case he doesn't want to be building Caustica, so it's better to augment his damage through critical hits + restricting their healing. T-rounds damage is magical so it synergises with your base attacks delivery anyway. Remember magic damage hurts most characters more because they have naturally low magic defence.

Core item: Plasma Blade. The amount of power converted from this item into your abilities covers for the loss of power later with items like OQuiver + Spectra, you want to build this early when you're heavily reliant on ability damage to win a fight.

Dust Devil Boxing is what makes immobile carries viable, that is; to hit and not get hit. Grim in particular slows his targets, with his he speeds up, able to run anyone down once he latches onto them. (You likely won't avoid too many hits however because his hitbox is still massive.)

Spectra is strong right now for anyone holding down their auto attack button, with items like OQuiver + Plasma Blade you will out trade anyone building more ability hybrid / crit focused items.

Onyxian Quiver is an item I used to think was hot garbage, but it has it's unique spot, in super high attack speed builds. It's not a great opening item, but once you're online it's a nice auxiliary effect to cleave like that. The strength of this item in an AS build is in the 20 on-hit combined with the speed. A lesser priority than Spectra for DPS, but can offer a nice trade off for sustainability with the small pocket of lifesteal.

The final item could be a few things, Equinox is a nice way to convert a high % crit + AS build into additional damage while adding some team fight survivability as you cut through the enemy team, but if you didn't want to go over your crit softcap with plasma blade you could go for sky splitter, which I recommend if they're running 3 high HP tanks, or even Absolution if their team is entirely composed of magical.