Drider

Hit Dice: If the base creature has fewer Hit Dice than the minimum drider Hit Dice for its size, it gains "aberration levels" until it reaches the minimum (use the "Creature advancement by type" table on page 13 of the Monster Manual). For this purpose, a fractional Hit Die counts as a whole Hit Die, so a kobold drider gains 3 Hit Dice for a total of 3-1/2, which counts as 4 Hit Dice. If the base creature already has the minimum Hit Dice, then Hit Dice are not changed.

Speed: Changes to 30 ft., climb 15 ft.

AC: Natural armor rises to minimum natural armor. If the base creature already has greater than minimum natural armor, it keeps that natural armor.

Attack: The drider gains a bite attack if it does not already have one. It uses its own base attack or the base attack of an aberration of its Hit Dice (as cleric). It can use its bite alone or as a natural secondary attack.

Damage: Bite damage depends on the drider's size. If the base creature already has a bite attack that deals more damage, it keeps that damage.

Special Attacks: A drider retains all the special attacks of the base creature and also gains spell-like abilities and a poisonous bite.

1/day -- dancing lights, darkness, detect chaos, detect evil, detect good, detect law, detect magic, faerie fire, and levitate. These abilities are as the spells cast by a sorcerer of the drider's sorcerer level (save DC 10 + Cha modifier + spell level). If the drider has no sorcerer levels, the abilities are cast as a 1st-level sorcerer.

Poison DC = 10 + 1/2 Hit Dice + Con modifier. The listed Strength damage from poison is both initial and secondary damage.

Special Qualities: A drider retains the special qualities of the base creature and gains Spell Resistance equal to 11 + Hit Dice.

Saves: The drider retains the saves of the base creature. If it gains aberration levels, its saves increase according to multiclassing rules. (Aberrations have saves as sorcerers.)

Abilities: Increase from the base creature as follows: Str +4, Dex +2, Con +8, Wis +2, Cha +2. The drider also gains ability score increases as a character would, according to its total character level. That is, a 2nd-level character who gains four aberration levels is a 6th-level character and gains one more ability score point at 4th-level.

Skills: The drider retains the base creature's skills. For each aberration level it gains, it gets +2 skill ranks plus its Int mod (but at least 1 skill point per Hit Die gained). Its class skills are Climb, Concentration, Hide, Listen, Move Silently, Spellcraft, and Spot. A drider receives a +4 racial bonus to Hide and Move Silently checks.

Feats: The drider retains all feats of the base creature. It receives extra feats as a character would, according to its total character level. For example, a 2nd-level character who gains four aberration levels is a 6th-level character and gains two more feats -- one at 3rd level and one at 6th.

Climate/Terrain: Underground

Organization: Solitary

Challenge Rating: Same as the base creature +2

Treasure: Double standard

Alignment: Always chaotic evil

Advancement: By character class

A drider's favored class is sorcerer, regardless of the base creature's classes. If this change in favored class causes a character to take experience penalties, then the driders appreciate the torture of a ruined life that much more.

The first example uses a 2nd-level hill dwarf fighter as its base creature. The second uses a run-of-the-mill goblin from the Monster Manual.

Large Aberration
Hit Dice: 2d10+10 plus 4d8+20 (59 hp)
Initiative: +1
Speed: 30 ft., climb 15 ft.
AC: 16 (touch 10, flat-footed 15) [+1 Dex, +6 natural, -1 size]
Attacks: Dwarven waraxe +6 melee and bite +1 melee
Damage: Dwarven waraxe 1d10+2/x3, bite 1d4+1 and poison
Face/Reach: 10 ft. by 10 ft./5 ft.
Special Attacks: Dwarven traits, poison, spell-like abilities
Special Qualities: Dwarven traits, SR 17
Saves: Fort +9, Ref +2, Will +5
Abilities: Str 15, Dex 12, Con 21, Int 10, Wis 12, Cha 10
Skills: Climb +10, Craft (weaponsmithing) +5, Hide +6, Jump +9, Move Silently +5
Feats: Cleave, Great Cleave, Exotic Weapon Proficiency (dwarven waraxe), Power Attack, Sunder
Climate/Terrain: Underground
Organization: Solitary
CR: 4
Treasure: Double standard
Alignment: Always chaotic evil
Advancement: By character class

Combat

Poison (Ex): Bite, Fortitude save (DC 18), initial and secondary damage 1d6 Strength.

Spell-like abilities: 1/day -- dancing lights, darkness, detect chaos, detect evil, detect good, detect law, detect magic, faerie fire, and levitate. These abilities are as the spells cast by a 1st-level sorcerer (save DC 10 + spell level).

Medium-size Aberration
Hit Dice: 4d8+12 (34 hp)
Initiative: +2
Speed: 30 ft., climb 15 ft.
AC: 16 (touch 12, flat-footed 14) [+2 Dex, +4 natural]
Attacks: Morningstar +4 melee and bite -1 melee, or javelin +5 ranged
Damage: Morningstar 1d8+1, bite 1d3 and poison, javelin 1d6+1
Face/Reach: 5 ft. by 5 ft./5 ft.
Special Attacks: Poison, spell-like abilities
Special Qualities: Darkvision 60 ft., SR 15
Saves: Fort +7, Ref +3, Will +4
Abilities: Str 12, Dex 15, Con 19, Int 10, Wis 13, Cha 10
Skills: Climb +9, Hide +12, Listen +5, Move Silently +10, Spot +5
Feats: Alertness, Dodge
Climate/Terrain: Underground
Organization: Solitary
CR: 3
Treasure: Double standard
Alignment: Always chaotic evil
Advancement: By character class

Combat

Poison (Ex): Bite, Fortitude save (DC 16), initial and secondary damage 1d4 Strength.

Spell-like abilities: 1/day -- dancing lights, darkness, detect chaos, detect evil, detect good, detect law, detect magic, faerie fire, and levitate. These abilities are as the spells cast by a 1st-level sorcerer (save DC 10 + spell level).