I run a campaign that has a strong Lovecraftian presence, but a few of the players in my current group are also pretty Cthulhu Mythos-savvy. Below is my attempt at fixing this, by having a few new entities, two of which I've put below.
When writing for these entities, I've tried to keep a Lovecraftian aesthetic without necessarily making them incomprehensible cosmic horrors. I find it's difficult to keep having inscrutable entities with unknowable goals remain compelling the fifth time in a single campaign.
Both entities have a relevant spell, though I wouldn't necessarily have them on a list players can choose from.
Ittravok, the Inevitable Shelter
To most people who see it, Ittravok is something that comes in visions, warning of an apocalyptic future. It tells them it can help save humanity, but they will need to cooperate with it. Ittravok gives magical knowledge to those who accept and guides them to others to form cults of Ittravok.
Each cult of Ittravok is essentially a group of doomsday preppers with serious magical abilities on top. They use the knowledge given to them by the entity to "sanctify" an area. The deal is, the sanctified land and everything on it will be kept safe by the Inevitable Shelter, forever.
Some cultists receive the spell Prepare the Way for Inevitable Shelter (see below).
Sometimes these isolated communities realise that they lack the manpower or resources for long-term survival. It is not unheard of for cults of Ittravok to kidnap people to swell the numbers of the prepper community or to add specific skills.
When Ittravok is finally invoked into our world, the entire sanctified is enveloped in a rapidly expanding cloud of multicoloured fog. When it fades away, it's all gone. Nobody knows where they go.
Besides the individual(s) it is visiting at the time, Ittravok can be seen by PCs who can see invisibility or through illusions, as well as those who employ spells like Detect Magic. Anyone with the spell Prepare the Way for Inevitable Shelter can be directly possessed by the entity. The entity speaks calmly and melodiously, using high-level enchantment spells to defuse hostility wherever possible.
Prepare the Way for the Inevitable Shelter - Level 1 Spell
This spell is a ritual used to mark a small area as "sanctified" to Ittavok. The old one promises that these sanctified areas will be saved when the time is right.
Having this spell in your mind opens yourself up to being possessed by Ittavok.
The Fossil in the Temple
It is likely that the Fossil in the Temple's true nature can be overlooked by players who do not know what they are looking for. The towering figure is extremely imposing, with two heads, four clawed arms, many tendrils all over its body and a pair of enormous scorpion-like stingers, but it's not something that will immediately concern the PCs.
That's because it is not a threat at the moment.
Its original name is forgotten. It is called the Fossil in the Temple because the entity itself is petrified and currently unable to do anything until it is released. Regular use of Stone to Flesh isn't enough- it requires its own spell to be found and brought here.
In my current campaign, the Fossil in the Temple was located extremely early in the game, in one of the first dungeons. Later, I introduced a spell which the players have given some serious thought on whether or not to use:
Release the Abomination - Level 1 Spell
This spell releases the Fossil in the Temple from its petrified prison when cast. The scroll that this spell is found on suggests that it may be possible to get the Abomination to agree to serve the caster for as long as a year and a day on the condition that it is placated with tribute, but this particular scroll offers no advice on what these tributes might be.
Once this spell is cast, it can be removed, because there is no other use for it.
My players have not yet decided whether or not they are going to release and attempt to bind it yet.