who can solve this toufh problem?

In one of Dorothy Sayers's mysteries, Lord Peter is confronted with the messageshown below. He also discovers the key to the message, which is a
sequence of integers:

787656543432112343456567878878765654343211234
3456567878878765654433211234

message:

I thought to see the fairies in the fields, but I saw only the evilelephants with their black backs. Woe! how that sight awed me! The elves danced all around and about while I heard voices calling clearly.Ah! how I tried to see-throw off the ugly cloud-but no blind eye of a mortal was permitted to spy them. So then came minstrels, having goldtrumpets, harps and drums. These played very loudly beside me,breaking that spell. So the dream vanished, whereat I thanked Heaven. Ished many tears before the thin moon rose up, frail and faint as a sickle of straw. Now though the Enchanter gnash his teeth vainly, yet shall hereturn as the spring returns. Oh, wretched man! Hell gapes, Erebus nowlies open. The mouths of Death wait on thy end.

QUESTION : Decrypt this message