Not really needed as I though I need to scour the map for the spawner like the 4th boss but turned way more easier than that, I decided to scout big piece of land next to where we built out base.

I usually scout the boarders to see how big the island is before giving it a go & it appeared to be it, what give it is that the water distancing the island is very short with other big pieces of land on our map are usually separated by huge sea.

Thanks for the info it means that I'll just have a grand expedition ahead of me.

It all depends on seed. I found the map stone in the first or second Plains biome that I visited. When in the Plains, you may find a small cluster of small stones with a large baddie guarding them. The rune stone is often there.

The actual spawn location is rather large and often I have found the boss spawn and not found one of the pointer stones.

I don't want to describe what it looks like due to spoiler, but if you want to know just ask.

every time Ive searched for the rune stone, Ive always ended up finding his altar instead... I have found the rune stones after the fight, so yes they do exist... Ive just never found them before the fight.

Yes there is but i found his spot way before i ever found the clue. the clue ended up being right next to where he is anyways.