Most will claim you can trigger it, but that's all talk for now. Others will say you can't but you can speed up their arrival by defeating each mini-battle between the Tuatha De Dannons and the Fir Bolgs. After 3-5 mini-battles, finally, another mini-battle will come, except this time, both groups won't be alone. The Fir Bolg will be accompanied by Eochai and the Tuatha De Dannans will be accompanied by Jack in Irons. That's where they fight along with their little lackeys.
From my last experience, it took a few hours of waiting until they came out. So be patient. As far as the devs, they have officially claimed that they're on a timer. That's what I go by as well. This all takes place at the Grim Vale.
Eochai seperately spawns by itself down by the pumpkin patches in south-west of the map. Jack in Irons seperately spawns around his thorn, south-east of the map, behind Salamanca.
Erm...sorry I'm new, I'm guessing this should be in Player Questions, this is obviously not a guide. So if anyone could move it, thanks again.
I've searched through Croatoa all day getting all the badges, except for the two monsters Jack in Irons and Eochai. Apparently and luckily, Jack in Irons appeared somewhere in the NE area and we were able to take him down. As for Eochai, I don't know where he spawns except for the point that vidiot maps shows, but I never saw him.
I've heard that there is supposed to be a huge battle in the valley every like 45 min and then they both show up. But that hasn't ever happened to my knowledge. Did the triggers change? Where is this guy?
Thanks