How do I regain entrance to the Soul Cairn?
TL;DR:
- As a part of the Dawnguard questline, you have an option to be soultrapped by Serana, allowing you to enter for the first time. This is not available after you leave upon the quest completion
- To enter again, you must become a Vampire Lord. You may need to "Rank up" to a level 2 Vampire Lord before being able to re-enter, but you should be able to re-enter at will.
From the wiki
One cannot enter the Soul Cairn without being a vampire or being partially soul trapped. Being soul trapped causes the Weakened Soul ability to activate, which decreases Magicka, health, and stamina while in the Soul Cairn. This can be negated after completing Chasing Echoes and talking to Valerica, who will instruct the Dragonborn to find the Soul Essence Gem. Upon doing so, all stats will return to normal.
Unfortunately I couldn't find any clear indications that you can become soul trapped again to re-enter. Becoming a vampire again seemed to be the only option, as Serana will no longer give you the option to become soul trapped again.
Alright I figured it out, if you become a stage 2 vampire it will let you in.