Dante the Deceptive is encountered in a Concealed Caverns event. Notably, you can only run into him after you've completed (not win) at least seven runs, and the last time you can run into him is on the fourth ring, as the event won't happen any further along. In the event, a mysterious figure tries to shove three Blight cards on you. Accept the "gift" and continue.
Two battles later, Dante will automatically reappear and offer to join you. You will receive Dante's Cloak (+2 Magic Power per Blight card), Dante the Deceptive (gain Multistrike per Blight card) and the achievement. I'd recommend prioritizing a stop at a Merchant of Steel to give him some more attack and turn him into a strong backliner.