Predecessor Builds > GRIM.exe > Hybrid Attack Speed

The fridge is back, and he's pumping out damage. You could go with a boring crit build, or you could put those magic power ratios to work.

Grim is much less mobile than other carries, which means it's harder to enter/exit/reposition in team fights and you'll have less agency to choose your targets (besides ult, of course). Instead, you'll want to focus on bursting back line characters if they get into range, but more likely you'll just be kiting the front line as they try to get on you. In lane, prioritize safely farming and poking the enemy carry with your bomb. Once the enemy carry is around 50%, you might be able to look for a kill if your support has good cc, but otherwise you'll probably lose a long engagement due to Grim's low base health and armor, and massive hitbox.

Build soul chalice on first back, then go magnify next. After completing first item, finish mindrazor, then proceed to other full items. I prefer Caustica 3rd, but always consider changing your build based on current game state.
-Mindrazor's max mana is all you need all game, and the physical power makes your autos punch even out of assault mode. Mid/late game you'll be able to clear a full wave with one bomb and a couple autos with the aoe damage.
-Magnify gives sufficient early attack speed while typhoon comes online, and allows your abilities to chunk the enemy carry for ~1/4 of their max health. Makes your finishing ults at the end of a fight hit extra hard against targets with low health/magic armor.
-Caustica allows you to fire in assault mode almost indefinitely, once the mana regen kicks in. Increases your flat damage, both autos and abilities. Also crucial if the enemy team has tanks and/or are building magic armor. 100% of Grim's damage is magical, so magic pen goes a long way.
-Prophecy rounds out your sustained damage, granting huge attack speed and damage. Always works well as 2nd item, but I find the magnify burst to have much higher kill threat in lane. Prophecy is better for prolonged fights.
-Ashbringer allows you to shoot bombs like nobody's business, but this does mean your mana will drain fast. Assuming consistent attacks, you'll have bomb up every 2-3 seconds. I wouldn't recommend building this until 4th or 5th item, and is especially potent with magnify.
-To go full magic damage, you can swap out ashbringer for oblivion crown or wraith leggings. Go crown if you want to do max flat damage, great for eliminating squishier teams, but wraith leggings will do more against tankier targets throughout a fight, due to the magic pen and <40% health damage amp.