[Alright, Attack of Opportunity doesn't work. Noted.]
Seeing the ball of darkness run a short distance away, and taking a little cold damage which slows him ever so slightly, the tortle sees no recourse but to run into the fray once more.
Having a lot of movement left, he runs to roughly the center of the ball of darkness, where he would presumably be bump into you, taking an attack from the dog on the way.
DC13 Str Save if the dog hits 1d20+10 : 20 + 10, total 30
He will then attempt to shove you prone.
Shoving a Creature
Using the Attack action, you can make a special melee attack to shove a creature, either to knock it prone or push it away from you. If you're able to make multiple attacks with the Attack action, this attack replaces one of them.
The target must be no more than one size larger than you and must be within your reach. Instead of making an attack roll, you make a Strength (Athletics) check contested by the target's Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). You succeed automatically if the target is incapacitated. If you succeed, you either knock the target prone or push it 5 feet away from you.
Blinded
A blinded creature can't see and automatically fails any ability check that requires sight.
Attack rolls against the creature have advantage, and the creature's attack rolls have disadvantage.
Shove does not mention that it targets a "creature you can see" so I will go with the assumption that this check does not automatically fail.
Athletics for shove 1d20+15 : 14 + 15, total 29
[Contest that with your Athletics or Acrobatics.]
Assuming you fail he will make an attack, disadvantage and advantage canceling out. If you do not fail, he will shove again.
Nunchuck Attack 1d20+10 : 15 + 10, total 25
bludgeoning damage 1d8+5 : 1 + 5, total 6
-OR-
Athletics for shove 1d20+15 : 3 + 15, total 18
[Con save DC 16 if the attack hits]
[I have more to do, but it depends on whether you are stunned or not.]