Astral Stalker

Predators: now in D&D!

Astral stalkers are LE, 12 RHD, medium-sized outsiders obsessed with hunting the ultimate quarry (a mindset that shouldn't be hard to combine with an adventurer's life). Right now they have a LA of +6, which is the very upper limit of what has ever been assigned in this thread. Given that astral stalkers lack overleveled cleric casting, theirs will probably end up a lot lower.

The stalkers' chassis is quite nice, with +10 natural armor, 50 ft. land speed, and bonuses of at least +10 to every physical stat as well as charisma. Wisdom and intelligence receive smaller boosts, but are not low by any means.

Astral stalkers have two claws that are about as strong as short swords (including the crit range), so not that incredibly useful. Sadly, a DM would be well within their rights to rule that their comically large claws render them incapable of wielding weapons, which'd be quite a problem for any martial astral stalker.

Their second natural weapon is a 'throat dart' which presumably is some kind of poisoned launchable spine, but isn't actually described anywhere in the text. While its damage is disappointing (1d4), the stalker's strength modifier does get added, and the paralyzing poison is a semi-decent SoL (albeit with a short duration of only 1d4 rounds). Note that these weapons can't both be used in a single full attack.

Other abilities of the stalker include 2d6 sneak attack, Camouflage (as per the ranger ability), Evasion, DR 10/magic, Unearthly Grace (hello there, +10 racial charisma!), and a couple of tracking-related abilities. First of these is Track as a bonus feat, second is Swift Tracker (which reduces the penalties for moving fast while tracking someone, and last is Elusive Prey (which imposes a penalty on the survival checks of anyone tracking the stalker). Note that the last ability technically applies to all survival checks: if you want to forage for berries while tracking an astral stalker your chances of finding something will plummet.

In the end, I can't help shake the feeling that the astral stalker is just a CR 12 monster trying to be good at something that stopped being relevant around level 6. Yes, it can track quite well (not as good as an 8th-level ranger, but still), but at a level where magical flight became commonplace several levels ago, 1d6 claws aren't a viable weapon, and a lot of things are immune to poison, sneak attacks, or both, you could be better off just playing a PC.

That said, the ability score improvements are quite large, Unearthly Grace is an amazing ability to have, and paralyzing poison can single-handedly win encounters. +0 LA seems like a decent compromise between the stalker's inability to do what it wants to do and the sheer force of its chassis.