Erebus Seals, is the new charm-like item of EftA. Charm-like item means that you can keep this item to your inventory to gain the granted bonuses it gives. Erebus Seals can drop in Magic And Rare versions and that is the end of the similarities with every other items in game and the Erebus Seals.
Affixes in Diablo II items have restrictions, some affixes that rolls in boots cant roll in weapons, others that roll in jewels cant roll neither in boots or weapons and the list is endless. Erebus Seals break all this restrictions and rules. From skills on struck, to plus to skills, from all resists to attack speed and all the combinations of stats you can think of, everything can roll in the Seals. Exception to this are the affixes meant only for unique items, the blocking related and the stack size ones. Of course all this exciting things can make the Seals overpowered right? Well, here is where the new mechanic of Carry Weight shines. Read about it here: Carry Weight mechanic.
You may wonder though, what will happened to the Charms? Fear not, Charms are present and strong. All of them (small, large grand) are dropping as Grand Charms. Affixes common to Erebus Seals and Charms are favoring the Charms, because those are the one holding the higher values of those affixes. Some Charms has a chance not to roll Carry Weight, making them even more attractive.
I've always thought that Champion Packs in vanilla and the mods where uninspiring, they where strong but their loot was uninspiring, mainly dropping potions isnt exactly something you care. So, mainly you will skip them. I could add some rares dropping, sure, but still that was meh... I thought, lets give a meaning to those special monsters and the hunt for Champion Packs in Escape from the Afterlife 1.54 - Paths edition was born, cause those Champions are the ones holding the Erebus Seals, this dark charms that can break the rules and gives a unique feel to your build.
I hope you enjoy Erebus Seals and this new type of farm, the hunt for the Champions!
P.S: The name "Erebus" comes from the greek mythological creature called Έρεβος. Erebus was son of Death and brother of Night.