A Octagon is a Polygon with 8 sides. It is purple in color, and has a dark purple border around it. It is 1.8 times larger than a regular Pentagon.
Octagons can be treated as a Pentagon that has 8 times more HP and gives about 7 times more EXP. It becomes a good source of EXP for Players above Level 30 who don't want to take the risk of the Pentagon Nest, when Pentagons and Hexagons don't give as much EXP compared to the amount of EXP required to gain a Level. Tier 2 Players may also have a hard time destroying Octagons.