Draugr Necromancer allows you to cast spells from among any cards in exile your opponents own with ice counters on them, including cards that were exiled by previous Draugr Necromancers
The more choke cards you stack, the more effective they become, so he's often played with those two cards, as well as Aragorn, Heir to the White City (3R38) , Consorting With Wizards (2R97) , Sting (1R313) , and occasionally Galadriel, Lady of Light (1R45) and Gimli, Dwarf of the Mountain-race (2P121)
The latter is mostly due to the lifegain, sure, but getting to bring creatures back even temporarily can be really strong
Thanks to the sheer efficiency of its mana-fixing, unsurprisingly, Mana Confluence is an incredibly popular card, especially in Commander
Neverending Torment You know that Call an ambulance, but not for me! meme