'Altars' are alien artifacts which were once believed to be used for 'summoning beasts.' It is now known that they were actually alien contraptions which transform any being into a 'Titan', 'Leviathon' or something else, depending on the species and build of the original being.
An Altar is activated by activating switches at five 'control points' which surround each altar. Once this is done, the five control points take five minutes to warm up, before activating, burning the symbol of what was once believed to be a pentagram into the ground. This, in turn, activates the main altar, where the intended being is placed, where the altar will then transform the intended being into another form. If one of the control points is deactivated while the being is in its 'Demon State,' they will transform back into their original state.
These are the locations of European altars
Albania - Tirana - Skanderbeg Square
Austria - Vienna - Augarten
the Czech Republic - Prague - Prague Castle or the river
Denmark - Copenhagen - Christiansborg Palace
France - Paris - Arc de Triumph
Germany - Berlin - Reichstag
Greece - Athens - Acropolis
Ireland - Listoghil
Italy - Rome - Vatican City
Spain - Barcelona - Sagrada Familia
Turkey - Ankara
Ukraine - Kiev - Independence Square
England - London - Big Ben
Scotland - Edinburgh - Melrose Abbey/Edinburgh Castle
Wales - Cardiff - Cardiff Castle
Northern Ireland - Giant's Causeway
There are other altars, but the Australian one is already taken by humanity, and the others are broken...