There are 4 stages of this world change, as you said you could easily see in which state of the world change is currently by asking "Thornfire" to any guide NPC
- First stage - "Countless firestarters are in their cells bellow Shadowthorn, but right now they are safely guarded."
At this stage, Shadowthorn will be in its normal state, and Elf Overseers are going to respawn outside each prision. To get to the new stage all the elf overseers as well as every single elf must be killed at least once. If these two conditions are met, it will change to the next stage at the next server save.
- Second stage - "Most guards and elves preventing the firestarters from breaking out have been slain. Shadowthorn is in danger of being set ablaze."
This message will appear on the same day that the previous message, but this one means the two conditions fulfilled, ensuring that it will change state to the next one.
- Third stage - "Shadowthorn burns, and the followers of the bog with it!"
As simple as it says, the shadowthorn would be on fire, at this stage is where people look for the crystal wolf to mount and they need to put out the fire to get into the next stage, which basically would be to "restart" the world change.
- Fourth stage - "Shadowthorn burns, but the Tibians have been successfully fighting the fire as well as the firefighters."
This messages means, that enough of the fire has been extinguished, assuring you that the next Server Save the Shadowthorn will be off fire and will return to its normal state.
Now, as you can see in order to find the Elf Overseer you must look for the first message of the stages but keep in mind that there are going to respawn only and only three elf overseer per server save, once they're killed they won't respawn again.
One option to hunt this elves would be to get disconnected at the Shadowtorn, right before the server save when the stage will get back to it's normal state, so you must check for the 4th message to be prepared to get the world changed reseted.
The other option is to login right before the server save and check for the messages and if you see the first one, run to the Shadowthorn looking for the elves, but I've had experiences where I found the dead bodies even 5 minutes after the server has opened.