My reasoning is that when the Tain shattered, Soulblighter took a shard with him and left the rest in Forest Heart. So, The Deceiver enters through ne shard and comes out in Soulblighter's camp as SB has taken the shard with him ever since. So, in the enemy camp, they started attacking the most powerful enemies, and Dec goes for Phelot. Phelot was badly injured and bound by The Deceiver. So he throws us the knife then escapes to meet Shiver and tell her that Dec has escaped and SB is headed for Tharsis. She believes him and takes him on herself. Therefore, when Dec meets her in "Shiver", he is there and makes his move. I think the level takes place near SB's camp from MOC (if you look at the maps, they share a corner). So Shiver had gone to his camp to see if SB was sill there and to see what had happened when we come on the scene nd her shade betrays her.
Phelot was The Watcher's shade in TFL and joined SB when he came back 60 years later maybe having fled with him to the East. After all, The Watcher 'dies' in the Dire Marsh so Phelot could have been in his camp and followed SB to the Devoid when he was turned back at the Gjol.
