deep dark dimension

In the ancient city. in the middel is there a portal. if you collect al the shards and get the music disk you can hear sounds of the deep dark dimension. so how do i open the portal you may ask yourself. there a padestals under the portal you put 1 item from al dimension so nether and end you put on the 2 left pendistals a dragon egg and a enderpearl en on the right 2 pedistals netherstar and a blaze rod. in the middel theres 1 more pedistal and there you put a beacon then the portal opens and you can go in. in the dimension you wil see wardens and other mobs and maybe a boss?