The movie is now out in the US so we can discuss that spoiler in detail. You have been advised!
spoilers:end of spoilers
In Earth-838 Mordo introduces the captive MCU Strange to the rest of the Illuminati: Captain Carter, Maria Rambeau as Captain Marvel, Black Bolt (played by Anson Mount), Reed Richards (John Krasinski) and Professor X. A flashback exposition showed that after they defeated their Thanos with the help of their Darkhold-corrupted Strange, he was deemed too dangerous to live and Black Bolt annihilated him with his voice. Back in the present, MCU Wanda via the body of Wanda-838 she possessed, penetrated their headquarters, and after Reed, Black Bolt, Peggy and Maria failed to convince her to abandon her quest, she killed them one by one, beginning with Black Bolt. After Reed warned her that he could destroy her with one word from his mouth, she calmly replied "What mouth?". Pan to Black Bolt with a now erased mouth, and when he tried his power it backfired on him and destroyed his head.
His costume is very comic book accurate and his wings are retractable.