Sam's undead which dodged the undead creatures of Tamas and let them pass through, even though he looked like he was at a disadvantage, didn't go towards Tamas as expected.
Because they focused on the dead bodies on Tamas'side. They threw the smaller bodies towards Sam and the larger bodies were being destroyed by them. Tamas was baffled for a second before he came out of that shock.
Sam didn't go after the necromancer directly, instead, he focused on the one thing the necromancer cannot afford to lose no matter what. The source of the undead.
Tamas immediately made a move, he created the shadow undead out of the bodies that are on verge of collapse from Sam's undead to preserve something. And then made these shadow undead to kill Sam's undead.
