Horn? You mean the Rhynn Lanthorn? (sp)
When you talk to Elhan after retrieving the artifact, the game will 'teleport' you to an area where you can gain entrance to the Elven City. It's automatic.
Are you sure you got the right item?
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
