The only advantage that Foul-Tongue Invocation has over this card is its “reveal a Dragon from your hand” clause. If you happen to reveal a Dragon from your hand when you cast it, you additionally gain 4 life. But outside of Esper Dragons, or any other deck that plays Dragonlord Ojutai, Icefall Regent, or Dragonlord Silumgar, it’s not the most relevant ability.
Overall, To the Slaughter would prove itself in a planeswalker-heavy metagame than Foul-Tongue Invocation. To the Slaughter found itself in the main decks of Esper Control, various Blue/Black and Green/Black decks, as well as many other sideboards. It always felt like a more playable card than Foul-Tongue Invocation. While gaining 4 life is a nice boost in the right context, having the additional ability to do away with a planeswalker made this card better in the long-term.