Deeproot Waters Whats more blue than merfolk
Tormod, the Desecrator is the classic pairing to generate board presence with your card advantage, but you could also try Glacian, Powerstone Engineer to facilitate the self-mill or Tymna the Weaver for even more cards
Reliable closures are also important to prevent accidental openings that could lead to lost cards
Being this large for this price at the time was an outstanding deal, even if nowadays it doesn't look like much, and the bonus life points were like adding insult to injury, causing players to wince looking at their hand that little bit more and do it again when you cast additional black spells
The cards you don't use right now can hold immense power that'll be able to boost your future strategies
Recurring threats that need to be exiled are usually very powerful and difficult to deal with for almost all decks, and Vengevine is a key piece in dredge decks since it benefits from self-mill and is one of the better payoffs for the strategy