From the Official LOTRO Forums:

https://www.lotro.com/forums/showthread.php?603653-Update-18-Beta-1-Release-Notes-amp-Known-Issues-PRELIMINARY-AND-NON-FINAL-BULLROARER-ONLY&p=7557689#post7557689

The raid is being built off of a solo instance that is in Update 18. In U18.2 that same storyline and space from the solo instance will be expanded upon with additional raid bosses, space and mechanics not found in the solo version you experience in U18. We made this choice because while this moment made a great raid experience, we did not want to block that part of the storyline from those who might never wish to do a raid.

https://www.lotro.com/forums/showthread.php?603653-Update-18-Beta-1-Release-Notes-amp-Known-Issues-PRELIMINARY-AND-NON-FINAL-BULLROARER-ONLY&p=7557657#post7557657

The raid version of the instance will be coming in 18.2 the next major patch after Update 18. Summertime.

Personally I am quite happy that the raid will not released with U18.0. The update in itself presents some new instances (two 3 mans and a 6 man), new avoidance mechanics, new mitigation mechanics, and a level increase. This is a lot all at once and would take away from the quality that they can put in to the raid. Turbine has not released a new raid since Erebor (U10 in 2013) and they need to get this one right. Fingers crossed that all hands will be on deck and this raid will be epic !!!