To me, he said, "Beware the whispers, for they whisper lies.
Do not wrestle with the demons of the dark,
Or else upon your mind they'll place a mark;
Do not listen to the shadows of the deep,
Else they'll haunt you even when you sleep"."
El-Harím was an unknown location somewhere in the Alagaësian world. It was a ruin at the time of the Rider War and was said to contain a danger so great that Murtagh and Thorn, equipped with their magic and the Name of Names, might not have been able to overcome it. It was the subject of a riddle that Nasuada used to repel mental attack.
The location is likely to play a future role in book V, or beyond, as Christopher said "You'll have to wait to find out" when a Reddit user asked why it was so dangerous. This suggests that we will find out, which may be some off-hand reference to a magical anomaly such as the crystal of Eoam, or perhaps it could cause the clash of light and darkness Christopher has hinted at before.