Kill the 3 carts you find across your way during chapter 5. In the "Furnace Valley: Valley Entrance", you will find a cart where you need to pull a rope device. Then teleport to "Woods of Ember: Ashen Pass I", you talk to Pale-Axe Stalwart, you exhaust his dialog. Then teleport "Furnace Valley: The Emerald Hall", you will find Pale-Axe Stalwart where you fought the last cart, you exhaust his dialog. Then teleport to Cooling Slope, 5th cart will be there.

i came back to correct my previous post go to the spot where you fight the last cart after the Coi fish boss go top and thalk to him several times then go to the Gate and he will be there whent true the howl act but yea you can go bk and do it .

i whent back and i know for a fact i killed all cart got the transformation the gate is lockt

>>4 no, brother, I've run across the whole chapter and after beating the final boss Pale-Axe is nowhere to be seen and the fifth cart doesn't spawn anymore, the gate is just sealed forever

>>3 you being dum-dum. I beat the chapter and went back and cart was there after talking to the bull NPC where this guides says it is. DOUBLE CHECK YOUR WORK DUDE!!!!

I can't believe this awful guide does not even mention the quest is MISSABLE IF YOU BEAT THE CHAPTER BOSS BEFORE CONCLUDING THE QUEST!!! The Rusty-Gold Cart won't even spawn! Terrible design choice from the developers, because at the moment the path to the boss is enabled, then you have to literally abandon everything to do a detour and fight the last cart. It is completely immersion-breaking!

Just a heads up that interacting with the dead cart is not actually required for this quest at all. That's for the horse quest. I got into the secret area without doing that step.

If you missed Ma Tianba before the dead cart quest, then you can still progress the cart questline, Just keep pulling the string at the cart and that will count towards quest.