Alacrán literally translates to "scorpion", but in Mexican Spanish, it is often used metaphorically to describe a dangerous or intensely troublesome person.
In the lyrics, the singer warns his ex by saying, "no te imaginas qué alacrán te echaste" (you have no idea what kind of scorpion you brought upon yourself). This vivid idiom means she has invited a massive, venomous problem into her life by crossing him.
Ever had a breakup so bad you wanted to get even? Well, El Tóxico takes that feeling and turns it up to eleven. This isn't a sad song about heartbreak; it's a hilariously over-the-top revenge plan. The singer feels he was the perfect boyfriend, declaring, 'Fui el mejor de tus amores' [I was the best of your loves], but his partner left without any explanation. Now, he's decided to give her twice as many reasons to hate him as he ever gave her to love him. His goal is to become her 'peor castigo' [worst punishment].
And he gets really specific about his plans. He vows to be 'tu ex, el tóxico / El innombrable' [your ex, the toxic one / The unnameable one], a nightmare, and even a toothache. The threats escalate from annoying to completely unhinged, promising she'll never find peace until one of them is dead. He even makes the wild threat, 'hasta el perro voy a envenenarte' [I'm even going to poison your dog]! It's a darkly funny song about embracing the role of the villain after getting your heart broken.