In 2018, Desmond met John Constantine at the bar where he worked and the two soon formed a relationship.
Sometime after, hunted by a powerful demon Neron, Constantine asked Desmond to leave town for his protection. Hearing about the danger John was in, Dez gave him his Medallion, which was imbued with a protection spell, before agreeing to Constantines' plan. However, Desmond didn't leave and instead made a deal to bind his soul to Neron's in exchange of the demon sparing John's life. Constantine found out about it when in an attempt to send Neron to Hell, he had to send Dez there as well. 
Several months later, after Constantine joined the Legends, they traveled to New Orleans during a mission, where John was confronted with Marie Laveau - great-great-grandmother of Desmond.
Hearing about Dez's fate, Laveau pressured Constantine to save "the last of [her] line" and he returned to the night they met with Charlie and had her prevent the two from meeting.
However, Desmond and John now met a few days later, leading to the assumption that their meeting is a fixed point in time, meaning they were supposed to meet and this event can't be changed.
Constantine then decided to travel back and break up with Desmond before he could fall for John, lying to Dez that he simply used a love spell on him and what they had wasn't real. initially, Desmond understood the situation as John trying to sabotage his own happiness and tried to talk him out of it, but at the end he left after claiming what they had was real, which caused the whole timeline to change.
Medallion- Desmond used to possess a medallion of the saint Marron, the patron saint of runaway slaves, which he later gave to John Constantine. The medallion is imbued with a protection spell and has been in his family for generations, once belonging to his great-great-grandmother, Marie Laveau.