Shell Retreat

Once per short rest, after the Tortle Monk is attacked, he can choose to pull from his inner strength to use his reaction and immediately retreat into his shell, gaining +4 to AC for all subsequent attacks against him (including in the same round).

While Shell Retreat is active the player may not make physical attacks or cast spells with anything but a verbal component. The player may emerge from their shell as either an action instead of using their movement that round. When the player emerges from their shell all enemies in front of the player and within 5' will gain an automatic opportunity attack. If the player's Dexterity is 17 or higher opportunity attacks are at disadvantage.

While Shell Retreat is active, if the player takes more than half of their total HP, Shell Retreat and all associated benefits end immediately and the player is Stunned for the rest of the round. The next round they must use their entire turn to emerge from their shell. Opportunity attacks are not at disadvantage when a monk is forced from their shell.

At 2nd Level the monk can use Shell Retreat up to twice per Short Rest by spending a Ki point to use the ability again.

At 5th Level all physical attacks against allies within 5ft of the Tortle are at disadvantage unless the attacker has reach. 

At 7th level the monk gains the Shell Spin action. The player spends a single Ki point, lets out a grunting cry (to warn allies to stand back) and extends their limbs just far enough to reach the ground. Then the Tortle spins their body causing all enemies to save vs crushing blow. Enemies making their saving throws are forced to retreat 5'. Enemies who fail take 3d6 damage, are thrown back 5' and become prone. Allies who choose to retreat from Shell Spin may move 5' away from the Tortle without giving opportunity attacks.

At 9th level physical attacks are at disadvantage against allies within 10' of the character unless the creature has Reach. The player may use Shell Retreat up to 3 times per Short Rest by spending 1 ki point per use after the first. 

At 12th level Shell Spin does 5d6 damage and saving throws are at disadvantage.

At 15th level Shell Spin counts as a magic weapon for the purposes of resistances and immunities. Shell Retreat has unlimited uses.