When Thanos (Josh Brolin) confronts Earth's Mightiest Heroes, the Guardians of the Galaxy and whichever other hero that wants to stop him come Avengers: Infinity War, it won't be about collecting the Infinity Stones as some sort of power trip. Instead, it'll all be part of the Mad Titian's "noble" crusade to fill his Infinity Gauntlet to reshape the order things, and in the process restore balance to the cosmos.
So far, it looks like he'll have the upper hand against all comers, especially with the Children of Thanos (aka the Black Order) and his Outriders helping carry out this cosmic conquest. Given that we've also glimpsed the Mad Titan himself beating down Iron Man (Robert Downey Jr.) and seemingly getting ready to unleash his rage on Steve Rogers (Chris Evans), well -- it really paints a bleak outlook for Earth's Mightiest Heroes.
However, amid the heroic factions and alliances formed against Thanos, there could be one secret weapon that the villain might not see coming: His adoptive daughter, Nebula.
In James Gunn's Guardians of the Galaxy Vol. 1, we were introduced to the character, played by Karen Gillan, as well as Thanos' other adoptive daughter, Gamora (Zoe Saldana). They were tasked with finding the Orb (Power Stone) for the villain, but Gamora scuttled those plans, defecting to the Guardians along the way. Nebula, on the other hand, sided with Ronan the Accuser, not just against the Guardians, but Thanos as well -- stemming from her hatred of how turned her into a warrior against her will.
Luckily for her, Thanos never found out about her traitorous actions, and by continuing to feign allegiance, she can remain close enough to betray him as soon as he lets his guard down. After all, Thanos is still under the impression that Nebula wants to kill her sister for turning on the family, a plotline that spilled over into Guardians of the Galaxy Vol. 2. Ultimately, Nebula realized the error of her ways, undergoing her own redemption by saving her sister's life. But because Thanos doesn't know about this, either, as far as he's concerned, Nebula's still his dedicated assassin.
Nebula can work this to her advantage, clouding Thanos' judgement with her loyalty so that he forgives her failures and keeps her by his side. Once she plays off his emotions (we did see Thanos holding a young Gamora's hand in an endearing flashback, after all) surrounding Gamora's treason, Nebula can easily set a ruse up where she promises her sister's head once and for all. This creates a window of opportunity for her to function as a true double agent, waiting to take her father out when least expects it. In GotG Vol. 2, she pledged "I will hunt my father like a dog and I will tear him apart piece by piece," so with this deception, we get the perfect setup for Nebula to make him pay.