Well pyke is unable to build bonus HP (similar to jhin with attack speed) but 14% of any bonus HP he gets is converted to AD. This means Stridebreaker gives him almost 80 AD total and a 90% slow that he can activate mid-dash in order to consistently hit the stun on his E. Anathema's Chains doesn't have AD on the statline but gives him almost 50 more AD (again, health->AD conversion) plus a personal exhaust on any enemy champ and a 20% longer stun at max stacks. You could really build normal pyke things like ghostblade or umbral glaive if you wanted to after these two items but the point of this is to create a 1v1 machine with the sterak's healing supplementing lack of lethality to scale up his grey health and black cleaver providing more damage thru the easily stackable armor shred + move speed. Hullbreaker is the new sanguine blade, so why not use it to split push and force a top laner to contest you while you're full build and kill them under tower? Build boots whenever you normally can in the order but the berserker's is there to make up for the fact that hail of blades has been exchanged for aftershock to make better use of the defensive stats, meaning you can 1v1 the ADC at any given point on two items with no fear of dying and giving over shutdown.