baleful > miserable, unhappy; deadly mansion > lodging, place of abode
3 In dreary darkness, and continual fear 4 Of that rock's fall, which ever and anon
ever and anon > always and straightway; hence: ever, continuously
5 Threats with huge ruin him to fall upon,
Threats > Threatens ruin > {The act of collapse of a building, etc.}
6 That he dares never sleep, but that one eye
That > [So that] one > [one remaining]
7 Still ope he keeps for that occasion;
Still ope > Ever open
8 Ne ever rests he in tranquillity: