The Justice League has faced all the DC Universe's greatest threats, from Darkseid and the Anti-Monitor to Perpetua and the Darkest Knight, but only one threat has managed to consistently beat both Superman and the Justice League at large - thankfully, it's one Wonder Woman is completely immune to.
The Justice League are no strangers to the dangers of the universe, they've fought literal monsters from outer space and defended Earth for as long as they possibly can. But there are some threats that give them more trouble than others. One of these threats is the Black Mercy, a deadly flower cultivated by the warlord Mongul. The Black Mercy is a plant that - when attached - lulls its victim into a hallucination of paradise, all while draining their energy. The Black Mercy forced Batman to confront the trauma of his parent's death, and it gave Superman the false fantasy of life on Krypton - in both cases, hallucinations they had to be saved from rather than escaping themselves. But one Justice League member is so strong that the Black Mercy isn't a threat to her at all.
Wonder Woman Is Immune To The Black Mercy
Readers got to see exactly how Wonder Woman was immune to Black Mercy in Trinity #6 by Francis Manapul and Emanuela Lupacchino. On a visit to the Kent farm, Batman, Superman, and Wonder Woman are ambushed by Poison Ivy. Ivy has the ability to control the Black Mercy and traps the three in a false reality created by the plant. Poison Ivy is able to beat Superman with ease, but this apparently doesn't work on Wonder Woman. While Batman and Superman both have to work through trauma to eventually see through the illusion, Wonder Woman is capable of just ripping the thing off instantly any time she wants. The only reason she stays in the hallucination is to protect her friends from manifestations of their trauma, escaping instantly once she knows they're safe. The fact Wonder Woman can just break out of the Black Mercy at any point means that she really has no problem with one of the biggest threats the Justice League has faced.
Wonder Woman's Truth Powers Have Unexpected Benefits
There are many deadly things in the DC Universe, but all of those threats can typically be fought either with brain or brawn. The Black Mercy is insidious because the victim doesn't even realize that they are in danger, which is why Black Mercy is so dangerous for the League. If Darkseid invades the planet, that's a clear threat to be taken down, but if Superman is trapped in an idealistic life on Krypton, he has no reason to try and escape. Wonder Woman is offered a fantastical vision of Steve Trevor by the deadly flower, but she in tune enough with her emotions that she isn't fooled, following her heart to lead her out of the construct.
A lot of comic readers overlook just how powerful Wonder Woman's ability to see truth actually is, but it's not just limited to the Lasso of Truth - she is able to wield that godly weapon because of her commitment to the cosmic ideal it represents. Wonder Woman is the embodiment of truth in the DC Universe, to the point where if she loses faith in it, reality itself begins to unravel. This quality has many unexpected benefits, and one of the greatest is her immunity to the Black Mercy - a deadly plant which has otherwise proved itself to be the Justice League's worst nightmare.