@PsiGuard True Bloodthirster can be replaced with items that give more defense, however don't forget about trundles ultimate. Bloodthirster just makes it so you lifesteal around 50-100 per hit.

Trinity Force is a great item on Trundle.

So is Frozen Mallet.

You shouldn't buy them both though.

If you're building Bloodthirster, you don't have enough defense.

@pluckin penguin thanks for the feedback, I can see what you mean by how trinity is a waste of gold, I have used trinity before in the past on trundle and it didn't go so well, I always ended up feeding. However by replacing it with Bloodthirster < test I manage to get a way better KDA ratio. I normally play normal games however I tried this build on ranked earlier and managed to get 9/3/13 in 1200 elo game (not exactly sure why im 1200 elo since im almost +20 W/L ratio in ranked) so I think that Bloodthirster is a pretty viable item for trundle.

I completely disagree with a Trinity Force on Trundle. Zeal is practically useless on trundle other than the movement speed, but if you're going to be buying a Force of Nature that will make up for it. Sheen's AP and Mana is wasted as well, only good part about it is the passive effect. So, if you do the math if you buy a Trinity Force you're wasting ~1930g out of the total cost for Trinity Force of 4070. Therefore, it is not needed on Trundle.
Now I agree though a Bloodthirster on Trundle is not the best choice. Also, I don't think it is worth it to buy both Warmog's and Frozen Mallet, you should get one or the other. You could consider getting a Maw on Trundle instead of a FoN if you're doing well. Solid Guide, so +1 for you. Hope I helped, have a good day!

@combatjack1 Yeah, I'm not too good at making guides for stuff, this really just focuses on the build.. I tried to explain how to use it but didn't explain too well xD

You need to explain your choices much better. Right now this guide is pretty much incomplete, because it's not nearly in-depth enough. A couple sentences on your item build is not nearly enough. You also need to add much more coding to this to make it look better, and therefore easier to read. Remember you can add a tooltip to any word describing an item, ability, etc. by encasing the word in double brackets. Ex. ((Trundle)) -but use brackets instead of parentheses, I'm doing that so you can see the formula. It will yield: Trundle.

Also, a Trundle without Trinity Force is a trundle that is not optimized. You benefit from every stat in that item and the Sheen procs immediately after casting Rabid Bite and seriously increases its damage. Bloodthirster is fun but not a good first item. Replace your Frozen Mallet with an immediate trnity force and you should be okay. I also think you should try 3x Greater Quintessence of Attack Damage as well in all areas you play, because it beefs up Rabid Bite that much more.