In the statium, what does the octagon looking thing mean with the C- in it ? Thanks

That means the dragon has gotten enough kills to have the rank C-. As they get more, they can rank all the way up to A+. This improves their combat stats, and makes a HUGE difference to their combat capability. Try to feed kills to your main fighters to get them to A+ ASAP for better results.

You can see more details (like the number of kills to reach each rank) on the stats screen for each dragon, just tap on the corresponding octagon.

Ok cool, thanks for replying.

@Mystery-Man Full explanation can be found here: https://forums.socialpointgames.com/topic/6311/april-2018-message-from-the-producers