Undead Fortitude makes Zombies really scary for a group for low level characters.
We fought 8 (in two groups of 4) in this past Fridays game. We lost our hireling Fighter in the fight with the first group of 4, and my cleric went down to 0hp. Luckily the rest of the party were able to kill the others before my cleric became Zombie Poo. If it wasn't for well placed flasks of oil, Fire Bolt cantrip from the Sorcerer, and everyone playing a fighting withdrawal; it could have gone pretty badly. Luckily the group decided to pull back and not open the final room (which contained the other 4 Zombies) so we could do a Long rest, the game could have easily ended in a TPK. Once we knew how to fight them, the second group was easier to take out. Still, we had to play it smart and use our brains. An all out fight, or had we been surrounded by them with no way to withdraw and the night would have ended badly. It was awesome! It's nice to be afraid of monsters again.
Was probably the best game of D&D I've played in quite some time. I'm really enjoying 5e!