After the death of Myrkul in the Time of Troubles, his divine domain was prematurely transferred by Ao to the newly-ascended Cyric, or "Cyruk" as he was known to Myrkulytes. Our dogma was unfazed by this change. A decade later when Cyric was defeated by Kelemvor in the City of Strife, the transition of worship from the Myrkulyte-Cyricists to the new god of the dead was just as smooth of a transition. Myrkul’s plans proceeded as designed with remnants of Myrkul's energy transferred into the nearby artifact, the Crown of Thorns. For all intents and purposes, the church of Myrkul was considered dead by the people of Faerûn. This was a time of showing that we understand the truth of death and the dead in the most intimate ways.
“(They were) merely changing the name on the letterhead and putting a purple sunburst around the skull outside the temples.” — Zeboaster "the Still Alive-For Now", on the conversion of Myrkulytes to Cyricists
However, that which was dead could not truly die. After Myrkul's death, a temple in his name was constructed beneath the Sea of Swords, in the area where Mystra’s newly revealed Chosen, a woman named Midnight, cast away the brown miasma that was released upon the destruction our Lord's avatar. This is our most holy and sacred site, requiring annual pilgrimage.
After Myrkul’s avatar was slain in Waterdeep in the Year of Shadows, 1358 DR, just before the majority of his power was transferred to Cyric during his ascension, his remaining divine essence found its way into the Crown of Horns which was hidden away in Blackstaff Tower. The ancient Netherese artifact held what was left of his personality and memories for a decade, after which it reformed its physical form on Toril. Over the next decade, Myrkul teleported the crown across the Realms, placing it upon the heads of many of his former devotees, both living and otherwise