I am currently renown level 18 on my shaman with the necrolord covenant.
I have completed all available campaign quests and I only have access to the first soulbind (Plague Deviser Marileth).
How can I fix this?
Many have found the same but I suspect it is a quest that you are missing perhaps.
See this post in another thread which links to the questline to open up Emeni as the second Soulbind.