Never tried it on Garen before but it shouldnt be too bad with decision strike and courage right? Has anybody confirm if this is a good or at least valid item on Garen?

what would you do with the mana it gives?

It doesn't seem like a bad idea on paper, but currently Garen is a lot more about doing damage without forfeiting tanky items, so Trinity Force isn't needed on him. You could get more useful stuff on him.

Garen scales poorly with AD, so that base damage you do isnt much worse than youd get with stacking IE+BF sword.
And TBH, his ability to stay in combat so long with sunflowers gives him more damage than most champions who arent top tier at what they do.

Carry Garen sucks. Get tank gear.

Btw, Trin is too expensive for this. Better get other stuff.

Not sure why you'd try other builds than tank/sunfire with Garen anymore. Only DPS item i get on garen is SotO and that's due to the active letting me chase and to build something from the brutalizer i may build early game if I'm dominating a lane. Then again, I'm not really a regular Garen player, but this is what I, and 90% of the Garens I see do.

Youmu's + Brutalizer + Atma's.

Everything else? Tank.

Garen doesn't need Trinity Force. He just needs people to face-check bushes more.