Read "The Power of Eld" as The Eldritch Dark:
Among the early poems by Clark Ashton Smith (CAS) that I have been reading, several took "the Past" as their theme and/or title. And in this sonnet, it is once again the Past that informs the tumultuous events described: "the Past's uncharneled dread / In swarming visions on the darkness burst".
The closing sestet goes on to describe all that the Past has unleashed in this apocalyptic dream, including this immaculate phrase: "Howlings of prophets mingled in mine ear / With death-lament of cities". This poem really highlights the dramatic, doom-encrusted writing that CAS is widely known for.