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.
If you thought breakups were all about crying into ice cream, Grupo Firme and Carin León are here to show you the petty side of romance. In "El Tóxico", the singer was left without an explanation after offering pure devotion: "Te daba mi vida, y te valió madre" ("I gave you my life, and you didn't care at all"). The Mexican slang valer madre means something was completely worthless to someone. So, instead of moving on gracefully, he swears revenge. He promises to become "la piedra en tu camino" ("the stone in your path") and "tu peor castigo" ("your worst punishment"). If he gave a thousand reasons to be loved, he will give double to be despised.
From there, the lyrics lean straight into over-the-top dark humor. He declares he will become "tu ex, el tóxico / El innombrable" ("your toxic ex, the unmentionable one"), acting like an endless toothache so that every time his ex says his name, it is only to curse him out by insulting his mother and grandmother ("recordarme a mi mamá y mi abuela"). He warns her with the Mexican idiom "qué alacrán te echaste" ("what a scorpion you threw on yourself"), which means she has brought serious trouble upon her own head. He even drops the outrageous, exaggerated threat, "hasta el perro voy a envenenarte" ("I'm even going to poison your dog"). Nobody is finding peace here until one of them drops dead or kills the other, making it an entertainingly dramatic anthem about becoming the villain your ex deserves.