"The connected" were constructed by a powerful mage experimenting with new forms of un-life. He made so that they can grow in numbers by a single one of them being left alone with a structurally intact corpse, which it will turn over the course of an hour into another one like it. The mage added a mental connection to the new beings which allows shared sight, thought, movement control and what little imprint of their souls remain in their flesh. In small numbers these are nothing more than simple zombies, but as their numbers grow so does their collective intelligence which is an emergent property of their numbers and the connection. By numbers their intelligence is "log 1.5(their number in a mile radius)".
The retained small imprint of their soul adds up in the collective. They get new skills by adding corpses thus sharing their skills. In rural areas after a massacre they will gain skills to work the field even if they won't use them but making roads might come handy. If they absorb the fallen after a battle they will become a force to be reckoned with as the once self-confident group of inquisitors that now form their ranks will attest. Absorbing a group of mentally ill patients will not help their cause.
As their intelligence and skills grow so does the diversity of their tactics. Initially they are little more than a small group of zombies fighting with their teeth and nails, easily defeated. But they will grow cunning, using big stones to throw, picking up weapons. Lying in ambush, unmoving until the prey are in position. Surrounding the foe and the ones facing him will concentrate on defense, the ones in their back will hack away, if he should turn they immediately change tactics forcing him to attack defending enemies while the ones attacking him will hit from angles where he can't defend properly. By reducing their numbers they will revert to more primitive tactics.
Taking full advantage of being undead they can for example form a boat by clinging to each other with one hand and "swimming" the boat forward with the other. They can form temporary structures to get over walls or getting their brethren up on the ceiling for an upcoming ambush. If their numbers go too high they will make contingency plans for example planting small groups of themselves with imprinted instructions to do. So even if the main bulk gets destroyed some will be lying in wait decades only to rise, find the nearest cemetery and start anew with new tactics.
They do remain walking corpses with the strengths and weaknesses of walking corpses. They require no sleep, air, food or rest but they cannot learn on their own, reproduce or heal. To destroy them you need to structurally damage them until the magic animating them can no longer be sustained. They take normal damage from slashing, half from blunt but only 1 damage from piercing weapons. They take double damage from fire or divine magic. Their goal initially is to destroy the living. They grow to want to survive and finally to absorb as much intelligence as they can. They can only learn new things by growing their numbers. So even if at first they will want to destroy the living in the end they are likely to farm them. Letting them get born, grow, learn, make children of their own then just kill them so they get absorbed.