Well, I needed a creature that could quickly kill the Yith...so this is my first pass at such a thing.

Using the Colour Out of Space as a pattern.

Time Eater

SZ S MV 20 DX 12 AC 9 HD 10 ATK 1/special D 1d6 ML 12

A many tentacled demi-material form with a distinct and coruscating glow. Each will be a
random color. Very rarely they are multi-hued.

Can only be hit by magical weapons.

Disintegration does not work because of the semi-corpeal
nature of them.

Casting time related heals them for 1d6+spell level hit points.

They feed on time. If a spell that manipulates time is "present" then they
are targeted. "Present" means if the spell is memorized or if there is an
innate ability that is time related.

These attacks do 1d12 points of damage. For each
point of damage done to defender that many spell levels are "lost"
to "time damage". The "time damage" is also applied physically.

For example, Temporal Accleration is a 5th level spell. When
5 points of "time damage" are done to a defender with that spell
memorized then that "time ability" is lost, as if it was cast.

If there are no other time related spells memorized then the time
eater stops the attack. "Time Damage" "attacks" from highest spell
level to lowest. Any "time damage" done carries over to the next
spell.

If the "time ability" is an innate quality then then time eater
keeps attacking until the creature is dead.

After not being attacked or not find any "time" to feed on a time eater will go dormant in 1d6 turns.

In their dormant state they have a fainter, more blurry glow. Any magic that can interact with them
will awaken them. Walking through them with magic armor or pendant will awaken them.