Note, I did take some liberty bending the rules to make this creature fit with my storyline. The rule-bending changes are: Stirges are "always Neutral", while this one is Lawful Evil, Stirges as Magical Beasts are not valid targets of an Awaken spell, but I used it anway to bring his mind out of the primordieal soup (a divine being cast it to create him), and hence created an Extraplanar subtype. Even though the speed increase for the Brobdingnagian template doesn't say so, I dropped the maneuverability of this creature down in the air, so it's on par with the dragons with similar fly speeds. I used the Animal progression table to figure out his new Attack bonus, which is actually less than it is supposed to be (a Magical Beast BAB would be higher).

For clarification, if I were to apply the Awakened template first (+2 HD, making 3d10 HD), then the Brobdingnagian (×16 HD) that would make 48 HD rather than 18 HD if the math were reversed, and as the Epic Bestiary suggests, the maximum hit dice for a Huge creature is 15, so 18 is even too high for this creature, plus he gained a Con bonus for his size increase.

Baalzebub       CR 17
Male Brobdingnagian Stirge
Lawful Evil Huge Magical Beast (Extraplanar)
Init +4; Senses Listen +4, Spot +4; Darkvision 60 ft.

AC 13, touch 8, flat-footed 13 (-2 size, +5 natural)
hp 180 (18d10+90 HD)
Fort +7, Ref +2, Will +1

Spd 40 ft., fly 160 ft. (poor)
Melee touch +21/+16/+11 (Attach)
Base Atk +13/+8/+3; Grp +21 (+33 when attached)
Space 15 ft.; Reach 0 ft.
Special Actions blood drain

Abilities Str 31 (+10), Dex 11 (+0), Con 20 (+5), Int 9 (-1), Wis 12 (+1), Cha (-1)
SQ low-light vision
Feats Alertness, Weapon Finesse(B)
Skills Hide +14

Attach (Ex): If a stirge hits with a touch attack, it uses its eight pincers to latch onto the opponent’s body. An attached stirge is effectively grappling its prey. The stirge loses its Dexterity bonus to AC and has an AC of 12, but holds on with great tenacity. Stirges have a +12 racial bonus on grapple checks (already figured into the Base Attack/Grapple entry above).

An attached stirge can be struck with a weapon or grappled itself. To remove an attached stirge through grappling, the opponent must achieve a pin against the stirge.

Blood Drain (Ex): A stirge drains blood, dealing 4d4 points of Constitution damage in any round when it begins its turn attached to a victim. Once it has dealt 16 points of Constitution damage, it detaches and flies off to digest the meal. If its victim dies before the stirge’s appetite has been sated, the stirge detaches and seeks a new target.