I've been wanting to read more about how people saw themselves and the world in medieval Europe, and one thing that has interested me is whether people believed in an impending apocalypse at any point - in particular, around the times of the crusades, and later, the height of the Black Death.
One person who stuck out to me was Count Emicho, who seemed to believe he would become the "last world emperor", and begin the Second Coming of Jesus after taking Jerusalem. Despite this sounding rather insane to me, he managed to gain a pretty large number of followers.
So, how did Christians in Emicho's time, and later during the height of the plague, think of the idea of the apocalypse? Was it considered at all imminent or something certain to happen?
Yes, apocalypticism in general starts to take root in western Europe in the eleventh century (although you can find some interesting earlier sprouts--the Pseudo-Methodius text that really kickstarts Antichrist mythology was translated into Latin in the 7th century; in the 9th century, Agobard of Lyons tried to get someone else (I guess he was lazy?) to write a treatise on the coming of the Antichrist; the monk known as Adso finally wrote that treatise at the end of the tenth century.
The Crusades play an interesting role in the development and entrenchment of apocalyptic attitudes. Christianity was always eschatological, always looking towards the Day of Judgment. But for Augustine and theologians following him, the "millennium" or eschatological age promised in the Bible ran alongside the rest of human history, rather than subsequent to it. The Crusades, which turned Jerusalem from the eschatological allegory for heaven into a historic, concrete reality for western Christendom, paved the theological way for 12th century thinkers and prophets to bring eschatology into the human historical timeline.
Some prophets, like Hildegard of Bingen, preached and wrote relentlessly about the coming of the Antichrist--but stressed that day was still far, far in the future (hence the need to reform the Church now, because there's still plenty of time for it to be strong and fight heresy). Others, like Joachim of Fiore and especially his followers, predicted the onslaught of the forerunners of Antichrist, Antichrist, and then the dawning of a new age (for the Joachites, the "Age of the Holy Spirit") within a century or so. (So reform the Church now, because you don't have much time.)
These two perspectives--apocalypse SOON, apocalypse EVENTUALLY--fight for supremacy throughout the later Middle Ages, but in an ever-mounting feverish atmosphere. By the fifteenth century, prophetic pamphlets forecasting disaster through Bible, bits of Hildegard and Joachim, natural disasters, signs in the heavens are THE hot seller of the new printing press. The early Reformation absolutely takes place under both sides' assumption that the other side is harboring the Antichrist who will cleave the Church (remember, Luther did not "leave the Church"; Luther believed the now-Catholic Church was no longer the real Church) and bring about the End of Days through war, famine, catastrophe.
A lot of the scholarship on medieval apocalypticism is unfortunately hyper-focused and kind of esoteric ("the Book of Revelation versus the 'little apocalypse' of Matthew in the sermones de tempore of Johannes Nider"). Bernard McGinn, one of the leading scholars of apocalypticism, has a couple of translated primary source anthologies that might interest you: Visions of the End in the Middle Ages and Apocalyptic Spirituality (for the excellent Classics of Western Spirituality series). He's also the author of a multivolume encyclopedia of apocalypticism, but unfortunately those books are print-on-demand so they are cost-prohibitive outside an academic library.