Need Some Lawful Assistance

[Ithamar]

I'm making an app for a PbP game that is level 20 gestalt.  There are 3 BBEG's, all of whom are undead (2 vampires & 1 lich).  My character is a Marut, which will take up one entire side of the gestalt.  It is the other side that I am having trouble with.  My backstory is that my Marut was originally tasked with destroying the lich, which everyone thought he did about 300 years ago.  But now the lich has re-appeared and my guy is being ordered to finish what he started.

So we have a creature of pure Law that needs to be able to hunt down and kill the lich and destroy his phylactery once and for all.  Not to mention dealing with the various other undead (and some living) mooks and whatnot.  (He will be part of a team though, so he does not need to accomplish this task solo.  Other teammates have not been chosen.)

I was originally going to just go with Cleric 20 (with Mechanus domain), but that is feeling a tad uninspired at the moment.  So I'm looking for suggestions on how to make this guy pretty much the embodiment of Law & Order.  Prestige class suggestions would be greatly appreciated.  Also, he has lots of decent immunities from being a construct, but his HP kind of suck and his Fort save is definitely lacking.  How can we help out with that?

I had considered using the UA druid that does not get wild shape but instead gets WIS to AC & Favoured Enemies.  If you strip away some of the nature aspects of it, the class makes a pretty good fit since it gets tracking, speed boosts, and a few more skill points than cleric.  I'd probably swap the animal companion for the elemental ACF in Complete Mage and use the few sub levels from Planar Handbook.  But still seems like it could use a bit more LAWfulness.

The Divine Magician ACF from Complete Mage was also somewhat appealing, but would it be worth it for this guy?

Pretty much any source is available, and the DM expects powerful characters but will avoid anything obviously broken.  Using Fractional BAB & Saves.  I've rolled stats that are not awful, but aren't great:  15-15-15-13-13-10 (drop the 10 since no CON score).

Suggestions?

[sirpercival]

Homebrew?

If not, Incarnate/Cleric/Sapphire Heirarch.

[Ithamar]

Valid Source Material: All WotC books. Any other source by review only. Homebrew on a case by case basis. No means no, please.

This is from the game ad, so may be able to get some homebrew allowed.

What did you have in mind?

Also, lacking a CON score, I didn't think meldshaping would even be an option.  Otherwise Incarnate of Law would have been great.

[Jackinthegreen]

You might be able to refluff the Undead Meldshaper feat.

[sirpercival]

If you can get homebrew approved, here's my version of Construct Meldshaper.

Alternative idea: some sort of martial adept.  Swordsage would work quite nicely, but if you can get homebrew approval there are a huge number of homebrewed adepts (many of them mine).

[Pteryx]

Annoyingly, law and chaos didn't get the attention in WotC published material that good and evil did, as near as I can tell; I've been combing for such things myself on account of a Lawful Good prosecutor-cleric who emphasizes the lawful side.  That being said, on the subject of both homebrew and Tome of Battle, if you go that route there's Quicksilver Aegis to consider, particularly if you can have it alongside the lawful side of Devoted Spirit.  -- Pteryx

[Quillwraith]

Incarnates can go for lawful abilities; or a binder.
Andras was lawful good, then lawful evil, and thus fits thematically, and I hear his powers combines well with Abysm. Naberious gives suggestion every 5 rounds, there's some others.