grisly > horrible, fearsome
2 Sucking the seas into its entrails deep,
entrails > bowels, inner parts
3 That seemed more horrible than hell to be, 4 Or that dark dreadful hole of Tartarus steep,
that dark dreadful hole > (Lake Avernus, a volcanic lake near Naples; near it was the cave of the Cumaean Sibyl, through which Aeneas descended to the infernal regions) Tartarus > (The underworld of Roman mythology, the infernal regions; the lowest part of these; hell) steep > (Qualifying the hole rather than Tartarus)
5 Through which the damned ghosts do often creep 6 Back to the world, bad livers to torment:
bad livers > [those who live badly]
7 But naught that falls into this direful deep, 8 Nor that approaches nigh the wide descent, 9 May back return, but is condemned to be drenched.
drenched > drowned
212.7