Testudo

4 or more same sized individuals, with the same feat and shields, create a nearly impenetrable scale-like shield wall infront an on top. With more individuals the coverage can also be on the sides and the back.

Benefits : 

+5 to AC.

+3 to strength checks and saves

missiles ,that are not of a lage mass, deal no damage to those in the formation. 

AOE damage from fire, water, ice, thunder, and lightning is halved on a failed save and one quarter on a successful save. 

Individuals or materials of one size larger can walk/sit on top of the formation with ease. Individuals in the formation need to do a strength check for individuals/objects that are 2 sizes larger. 3 sizes larger is an auto fail.

Save for 2 sizes is a strength check of 16

Individuals who are 1 size smaller than the individuals forming the testudo can be in the testudo and gain the benefits and drawbacks (exceptions: smaller individuals' can attack once per turn from inside the regular and spiked testudo. If it is a ranged attack no disadvantage. If Melee it is made with disadvantage.)

Movement: 10 ft. No dash

Drawbacks: no individual in the formation can make any form of an attack unless the formation is changed to a spiked testudo. In which the benefits of the formation to AC, strength save and checks are reduced by 1. The melee attacks made from the individuals in the spiked testudo have disadvantage. The spiked testudo has no immunity to ranged missile

while in or forming a spiked testudo, if the order to brace is made the butt of the polearm into the ground and brace for attack. If an enemy enters your reach you get an opportunity attack dealing half damage. (UNLESS enemy is mounted) in that case there is no disadvantage on attack rolls and damage is doubled to the mount.

Cannot dodge or disengage until formation is broken.

Takes 1 bonus action to break formation or to change formation