Whenever Gwenom attacks, until end of turn, you may look at the top card of your library any time and you may play cards from the top of your library. If you cast a spell this way, pay life equal to its mana value rather than pay its mana cost.
Suit up Gwenom and swing in every turn, using her attack trigger to cast spells off the top of your library by paying life instead of mana. Deathtouch plus lifelink makes her a safe attacker who refills the life you spend, letting you chain cheap-to-cast threats and value spells into explosive, mana-free turns.
Lifelink and deathtouch make her a resilient, evasion-light but trade-proof attacker.
Effectively doubles your spell output by letting you cast off the top while still playing from hand.
Paying life instead of mana lets you cast high-value cards even when mana-screwed.
Lifelink gains back the life spent on cheap spells, creating a near-free engine loop.
Mono-black gives access to the best tutors, ramp, and recursion to set up the top of your library.
She must attack to get value, so she's vulnerable to removal, fogs, and pillow-fort blockers.
Casting expensive bombs costs huge chunks of life, and you only gain it back if she connects.
Heavily commander-dependent; removal-light tables can chip your life total down fast.
No card advantage if Gwenom is dead—the deck stalls without her on the battlefield.
Top-of-library access is hidden information you can't bank, so a stranded land 'wastes' the trigger.