[Pitchwife]

I was reminded of Utumno as well reading the first stanza, and Angband too; also of the 'bitter cold immoderate' wherewith Melkor made war on Ulmo's province in the Ainulindalë (but couldn't destroy its beauty).

If I remember my Blake well, Urizen is a spirit of measuring, limiting, constraining (which is why his name is reminiscent of horizon as well as the more obvious Your Reason, and he is iconically portrayed with a pair of callipers), which fits the immobilizing cold of winter described in the poem but makes him kind of an antipode to Melkor, the spirit of rebellion and extremes. Both are tyrants, but Urizen's tyranny is one of scientific positivism, political absolutism and religious orthodoxy - in short, he's the patron saint of all Blake's pet peeves.

All this has to do with the difference in Tolkien's and Blake's religious views. Eru Ilúvatar is, shall we say modelled? on the God of Christianity in whom Tolkien believed himself - a benevolent creator, rebellion against whom could ultimately only end in self-destruction; whereas in Blake's eyes the God worshipped by his Christian contemporaries was Satan the Accuser and rebellion against him absolutely necessary. Pullman's war against the Authority in His Dark Materials is pure Blake.

Since the thread starter mentioned Orc - yes, he is the spirit of 'rebellion, war, ferocity and slaughter', but this is only his fallen form, taken in reaction to Urizen's tyranny, and in his unfallen form he is Luvah (think of English lover with a northern accent!) - something like Eros or libidinous energy. In any case he's the most dynamic and kinetic of the Four Zoahs. There is nothing quite like him among Tolkien's Valar, except maybe - Tulkas?