Here is a possible way to spread out the Abomination:
Yuan-ti Abomination (sorry about the formatting; KEY: when you see a line of numbers with + in front of them the first is your BA, second, third and fourth are your Base saves Fort. Ref. Will respectively, and the last is your skills, if it says N/A you do not gain skills that level; also if your HD is the same you do not gain a con. bonus to HP AND you do not roll an HD. Example: at lv. 2 you have 2d8, and you gained an HD, at lv3 you still have 2d8 and you do not gain an HD or the benefits thereof)):
Level HD BA Fort. Ref. Will Skills
LVV1 1d8 +1 +0 +2 +2 (4X2+Int.)
Special: Medium Size, Darkvision,+2str., +2cha., +1 NA (natural armor), Spd. 30ft, Alertness (racial bonus feat), Feat (regular gained by HD)
Can take ten on climb and swim checks even in duress, +8 racial bonus to swim checks to avoid hazards,
proficient with all simple weapons, scimitars, composite longbows, bite, proficient with light armor (although they aren't wearing armor their half-blooded fellows do and they are wearing a shield so...), and shields (except tower shields),
Bite 1d8 (no poision yet; since the Large Yuan-ti's bite is 2d6, and it doesn't have improve natural weapon, the medium Yuan-ti's bite will do 1d8, one step below; you could always start out at 1d6 peg it up to 1d8 around lv 3 or 4 then once it gets large, I choose at 5HD lv. 8, it gets 2d6, plus any feats of course).
Lv2 HD BA Fort. Ref. Will Skills
Lv2 2d8 +2 +0 +3 +3 (2+Int.)
Special: +2 con., +2 wis., detect poision (at will; Cl=6), Alternate Form 1/day
Lv3 2d8 +2 +0 +3 +3 N/A
Special: +2Int., Swim 20ft., Lesser Yuan-ti Powers 1/day (all of the Yuan-ti Abominations, MM1 pg 264, at will spell-like abilities are its "lesser powers": they are animal trance and entangle; right now you can only use them 1/day. ALL of the yuan-ti's spell-like abilities [even ones they gain later, "greater powers", have a caster level of 1+HD from yuan-ti monster class; DC=10+Cha. Mod.+equivalent spell level).
Lv4 3d8 +3 +1 +3 +3 (2+int.)
Special: +2 str.,+2 Dex., +3 Natural Armor, Climb 20ft., FEAT (regular), Poison 1d2 con. damage initial and secondary (DC=10+1/2HD from this class+Con. Mod.;if you hit with bite you can poison), Alternate Form 1/day (same as the description in the book except you can only use it 1/day), Chameleon Form (like what it says in the book).
Lv. 5 3d8 +3 +1 +3 +3 N/A
Special: +2 con. +2 Wis., Blind-Fight (racial bonus feat), Improved Grab (like in book), Constrict (like in book except does 1d4 damage, increase to 1d6 when turn large).
Lv.6 4d8 +4 +1 +4 +4 (2+int.)
Special: +2Int., +2Cha., +2Str., Aversion (like in book; DC=10+1/2HD from this class+Cha. Mod.), Alternate Form (At-will), Lesser Yuan-ti Powers 3/day, Produce Acid 1d6 (like in book except does 1d6 acid damage and 2d6 in a grapple; if this costs a standard action to perform consider lowering the level this is aquired),
Lv.7 4d8 +4 +1 +4 +4 N/A
Special:+5 Natural Armor, Spell Resistance 4+HD from this mosnter class only (HD from other classes doesn't increase your SR, although I would house rule it does, such as like the Drow's; maybe its in the rules even), Medium Yuan-ti Powers 1/day (Deeper Darkness (lv3), Neutralize poison (lv4), Suggestion (lv3); CL=1+HD; DC=10+Cha. Mod. + spell level equivalent)
Lv8 5d8 +5 +1 +4 +4 (2+Int.)
Special:+2Str., +2Con., +2Wis. Poison 1d3con.
Lv.9 5d8 +5 +1 +4 +4 N/A
Special: +2Int., Large Size (among the other benefits and penalties, the most notable is that your bite increases to 2d6, and your constrict increases to 1d6; ability increases have been factored in to the progression), Lesser Yuan-ti Powers: At-will, Produce Acid: 2d6 acid damage (4d6 in a grapple)
Lv10 6d8 +6/+1 +2 +5 +5 (2+Int.)
Special: +7 Natural Armor, FEAT (regular) Spell resistance: 9+HD from this monster class (see above at lv7 though), Medium Yuan-ti powers 2/day
Lv11 6d8 +6/+1 +2 +5 +5 N/A
Special:+2Wis., +2Cha.
Lv12 7d8 +7/+2 +2 +5 +5 (2+Int.)
Special: +2Int., Poison 1d4con., Produce Acid 3d6 (5d6 in grapple; just like in book)
Lv13 7d8 +7/+2 +2 +5 +5 N/A
Special: +10 Natural Armor, Medium Yuan-ti Powers 3/day
Lv14 8d8 +8/+3 +2 +6 +6 (2+Int.)
Special: +2Wis.
Lv15 8d8 +8/+3 +2 +6 +6 N/A
Special: +2Int.
Lv.16 9d8 +9/+4 +3 +6 +6 (2+Int.)
Special: +2Cha., FEAT (regular)Poison 1d6Con. (both initial and secondary),
Greater Yuan-ti; powers 1/day (Baleful Polymorph lv5; CL=1+HD; DC=1-+Cha. Mod.+Spell Level; consider giving the class the Powers earlier, lv16 is pretty high for 1/day Baleful polymorph; maybe lv11 or 13; With this template you can rearrange the progression of some things such as the poison, poison is pretty strong, especially Con. damage, so I gave the full 1d6 at lv16, but maybe you could start the poison progression at 1d3 (lv4) and get 1d6 at lv12 (1d4 at lv8), notice I just increased it every 4 levels)
Also I would consider swapping two points of the LA for racial HD; it would have 11HD by the end which would give it three attacks. It would gain +2 BA, +0 Fort, +1 Ref +1 Will, it would get 2X2+Int. in skills, your SR would increas by 2, and your CL would increase by 2, you would not gain another feat . So its not to overpowered to swap the LA for the RHD. Also, A monster class does not gain ability score increase every 4HD, like standard PC's, however you can always houserule that they do (although the ability increase's they get normally factor this in its not to bad giving these classes a couple more ability points)
Hope this helps if you have any questions ask...
