Deck Idea

This deck plays like a traditional control deck--remove your opponent's threats while making it difficult for them to create new threats. Instead of using Kazakusan as its finisher, this deck uses Abyssal Curse and Curse of Agony for its win conditions. In general, this deck wants you to maintain a large hand size so that you get the most value out of Spice Bread Baker and Goldshire Gnoll. Since so many aggro/midrange decks can continuously make board-floods, this deck uses a ton of healing to let us be greedy with our removal. Baiting your opponent into overcommitting their board is the most important part of this deck. If your opponent tries to play conservatively, you can punish them by filling their hand with Abyssal Curses.

Synergies

Every spell in this deck is a Shadow spell, so Tamsin has synergy with all of them. She doubles the healing, removal, and curse output of this deck, so she is probably the most flexible card in the deck. Her best use is matchup dependent, so you'll need to learn what strategies and combos to go for with her for each game. For example, against control/combo decks, doubling curses (of either type) is probably what you need to use her for; against, aggro and midrange decks, you'll most likely need to double your healing and removal using her.

Brann is very flexible in this deck, and allows the deck to output a lot of extra healing and damage. When combined with Spice Bread Baker, you can heal half of your health very consistently. When combined with Sira'kess Cultist or Za'qul, you can double your Abyssal Curse output, make your opponent's hand much more clunky, and cause them to overdraw cards more easily.

Thalnos and Mistress are great to use in the early game for healing and removal. Thalnos can help to change certain breakpoints against aggro decks when combo'd with Grimoire of Sacrifice, Drain Soul, etc., and the added card draw doesn't hurt either. Mistress works well against aggro as a body to fight for board, but she also helps against non board-based decks as a target for our removal spells to heal or give our opponent Abyssal Curses.

Flex Cards

Currently, I have Rustrot Viper and Smothering Starfish in the deck to tech against the current meta; however, these are the least important cards in the deck if you are looking to add other cards or feel the deck is missing something. Personally, I think if you want to sub either of them out, then the best replacement card would be Full-Blown Evil so that the deck has some more flexible removal tools. Other cards to consider in place of these two could be Doomsayer, Queen Azshara, Zola the Gorgon, Blademaster Okani, School Teacher, Ambassador Faelin, or even Lord Jaraxxus. Most of these are meta-dependent, so should buffs/nerfs come at some point, you might want to keep an eye on these options.