Lake of Nightmares

Wychlarans and berserkers v alseids, alseids turn into shaaran centaurs and are no longer fey once the lake is corrupted.

Wychlarans didn't successfully imprison eltab. They just think they did. Eltab actually was influencing them into throwing him into the lake of dreams because he wanted to take the elvish pure magic of evermeet. He *led* the wychlarans here.

The wychlarans toss the 'imprisoned' eltab into the lake, he travels through and pops out the other side like he wants- but Xav makes the ultimate sacrifice and curls around it and dives back into the lake and imprisons eltab in the lake of dreams, his domain of delight, it turns into the lake of nightmares.

Xav confronts the witches with their naivety and arrogance, they start their ritual of imprisonment - weakened from having imprisoned eltab - he tries to get back to his lair before they can subjugate him. They successfully pursue and subjugate him in his lair- then force him to become a dracolich through the painful and terrible process of the cult of the dragon over the course of days.

Though his domain corrupted, his connection to the sharawood severed, he guards eltabs prison for 181 years.

- The feyhir (fyhir in 3e, iirc) actually coalesces from peoples' fear.

- An aboleth or two (possibly reskinned), with sahuagin or similar Deep-Ones-esque minions might make work well in a coastal area, possibly as the focus of a cult.

- Gibbering mouthers or chaos beasts have a definite Azathothiness about them, especially if they lurk alone deep in a cave somewhere.

- For some reason, phantom fungus always seemed very Mythos to me, especially if given an Int score and converted to the aberration type.

- The (low-power, non-epic) pseudonatural template from WotC's "Lords of Madness" is nice for slapping on mundane creatures to convert them into Things That Should Not Be. Or even the lowly fiendish, half-fiend or half-dragon templates could be easily reskinned to create malevolent beasts-- like warped, betentacled cave bears or something-- that could be worshipped by savages.