The following is likely to contain spoilers for non-GM TDE players:
The Nameless One's breaking out of his heavenly prison and subsequent rise are likely THE major theme of The Dark Eye 5E's meta-plot / "Living Story" ("Lebendige Geschichte" in German, don't know if that's the proper term in English).
This will be clearer once the "Aventuria Almanac" is released in English, which contains a lot more background information about different aspects of the game world - e.g. the pantheon of gods. Adventures with a high "Living Story" score* so far have been related to this, as was the first short story collection published. Currently, the "Starfall" ("Sternenfall") is happening - literally stars falling, star constellations moving and changing - which is interpreted to be a sign of the Nameless One tearing on his chains.
Therefore, if you have any intention of following the official content: just sit tight and you'll probably get all the Nameless One action you wish for. Since your heroes failed to stop that one particular ritual, it might even add a feeling of responsibility for what's happening. Be aware that if you homebrew a campaign revolving around this theme, you'll likely be incompatible with the official meta-plot.
With that out of the way, the most likely approach is to have disciples of the Nameless One - who obviously see these signs too! - become more active and work towards his return, just like in the adventure you already played. This could involve:
- Rituals to supposedly strengthen his might
- Undermining the authority of the Twelve Gods
- Intrigues to secure positions of power to deliver him a realm once he returns
- Subvert the order of society given by Praios (by inciting revolts or turning authorities against one another,...)
- There also is a tome devoted to the Nameless One, the "13 Encomia(?) of the Nameless One" which will drive readers to worshipping him.
- You can have stories of people trying to satisfy their greed, or who are shunned by society, who turn to the Nameless One to further their cause.
- Mad "scientists" who try to break His chains by physical or arcane means.
Basically, the whole array of "Evil Deity is gathering Its strength and Its followers cannot openly show, quite yet" clichés.
Quoting from the Aventuria Almanac (translation is my own):
The aspects associated with the Nameless One are Power and Domination, but also Egoism, Vengefulness, Temptation, Lies, Deceit, Treachery, Hatred and Destruction and even Self-Mutilation.
His followers and disciples like to hide behind a mask of righteousness, but secretly praise the downfall of the established order and hope for an important role in his new world order.
The prophecy of unlimited power and a thirteenth age under his rule seduces many to his dark doctrine, among them decadent nobles, unscrupulous patricians, hate-filled mercenaries or oppressed serfs, who are certain to deserve better.
The priests of the Nameless One are known for their persuasiveness and manipulation skills. Many of them are excellent deceivers, and help the cult with undermining or even toppling communities of faith, temples, rulers or even entire realms through intrigue, murder and dark miracles.
*In the German edition, adventures have a score between 1 and 4 assigned to them, depending on how much they influence the "Living Story". Apparently there is no such score in the English edition.