Guide to Secret
No mage is an island
Once you have levelled up your blink around 3 levels you should be able to walk blink from the bridge to the island that can be seen towards the North. Blink again to the next two islands that appear and then blink inside the pool. You will get the achievement, plus an extra skill point
Shoot down all squirrels on the map. A sound will play indicating that the secret is available. On the island to the West on the southern-most point -where a petrified squirrel used to be- a new portal will have opened. Go through it and blink inside the pool to get collect the secret and extra skill point.
What are you even doing here?
There is an island to the North of Exekias’ hut. There is no way to reach it -without using exploits-. You need around 12 blinking distance to reach it from the corner of the hut. That means that even if you got all the possible skill points available you still would not reach it. Therefore unless there are other means you can’t reach the island without exploiting.