Hydra

Diablo II	Notation	Basic	Class	Skill	Item	Ally	Quest	Area	Monster	Tale

1
2

Fire Bolt
Warmth

3
4

5
6

7
8

9
10

Type	Long range triple pet cast Fire medium range single target bolts
Tier	6
Required level	30
Required skills	Enchant
Requirements	None
Left button	No
In town	No
Mana cost	19.5 + (0.5*slvl)
Attack no mana	No
Weapon damage	No
Synergy skills	Fire Bolt, Fire Ball

Level01
6
12
18
24
30

This skill enables the Sorceress to summon a beast of pure flame from the core of the world. The Hydra have been allies of the Zann Esu women for generations, owing their freedom in ancient times to the actions of the renowned Sorceress, Habacalva. They never fail to answer a call from a proven Sorceress, and stand ready to spit bolts of scorching magma at her enemies.

Detail

Stat	Level
Stat	1-8	9-16	17-22	23-28	>28
Minimum Fire Damage	(5.5*slvl) + 8.5	(7.5*slvl) - 7.5	(9.5*slvl) - 39.5	(11.5*slvl) - 83.5	(13.5*slvl) - 139.5
Maximum Fire Damage	(6.5*slvl) + 13	(8.5*slvl) - 3	(10.5*slvl) - 35	(12.5*slvl) - 79	(14.5*slvl) - 135

Stat	Value
+% Fire Damage	3 * (Fire Bolt.blvl+Fire Ball.blvl)
Delay (Seconds)	1.6
Duration (Seconds)	10

Although summoned, caster's +% Fire Skill Damage from Fire Mastery and items, and -% Enemy Fire Resistance are applied (as are any synergy bonuses from the Sorceress' skills).
Each of the three hydra heads fires independently, so it is possible for them to fire at up to three separate targets.

Each hydra head fires bolts with ~14 2/3 yard range. Awareness range of hydra heads is slightly larger, so they can begin to fire on an approaching enemy while it is still out of range, hitting it as it comes into range.

12 frame attack animation which cannot be increased or decreased in any way. [1] 15 frame AI delay in all difficulty levels lengthens the minimum interval between consecutive attacks to 27 frames. Although theoretically this means each head could fire maximum 9 [250/27] bolts in 10 seconds (250 frames) for 27 bolts from all three, in practice pauses between shots mean that the average total is closer to 22 bolts between the three in 10 seconds, or 2.2 per second.
When cast continuously, this number of bolts per second is multiplied by duration / cast rate to calculate maximum fire damage per second after the first Hydra has expired. Cast rate is Delay (1.6 seconds or 40 frames) plus Action frame, so Sorceress cast rate is 40 + (4-7) or 44-47 frames, depending on Faster Cast Rate. For example, maximum fire damage per second after 10 seconds for a Sorceress with 45 frame cast rate (63-199 FCR) would be fire damage multiplied by 2.2 * 250/45, or ~12.2.
Hydra cannot be targeted by hostile units but will expire at end of their duration.
Useful for scouting and attacking targets not within caster's line of sight: cast at edge of screen and hydra heads will fire at any hostile units beyond within their awareness range; cast beyond a corner or doorway and they will fire at any hostile units around it.

Source

Council Members can cast Hydra, although they only apply 17% damage to players.

Skill bonus

+(1-3) Hydra (Sorceress Only) can spawn on ilvl >36 orbs and staves.

Oskill

Value	Source	Min Rlvl	Ladder
+10	Dragonscale Zakarum Shield	80	No

Can only be equipped by Paladin.

Charges

Level	Charges	Source	Rlvl	Ladder
6	36	Hexfire Shamshir	33	No
30	10	Hellfire Torch Large Charm	75	No

Chance to Cast

On Striking

Level	% Chance	Source	Min Rlvl	Ladder
15	12	Dragon body armor/shield	61	Yes

When Struck

Level	% Chance	Source	Min Rlvl	Ladder
6	3	Tiamat's Rebuke Dragon Shield	38	No

Hydra Shield suffix also applies Chance to Cast Hydra When Struck.

Level	% Chance	Item	Min Rlvl	Min Alvl
3	10	Amulet	30	40

Levels 1-60

Stat	Level
Stat	1	2	3	4	5	6	7	8	9	10
Mana Cost	20	20.5	21	21.5	22	22.5	23	23.5	24	24.5
Minimum Fire Damage	14	19.5	25	30.5	36	41.5	47	52.5	60	67.5
Maximum Fire Damage	19.5	26	32.5	39	45.5	52	58.5	65	73.5	82

Stat	Level
Stat	11	12	13	14	15	16	17	18	19	20
Mana Cost	25	25.5	26	26.5	27	27.5	28	28.5	29	29.5
Minimum Fire Damage	75	82.5	90	97.5	105	112.5	122	131.5	141	150.5
Maximum Fire Damage	90.5	99	107.5	116	124.5	133	143.5	154	164.5	175

Stat	Level
Stat	21	22	23	24	25	26	27	28	29	30
Mana Cost	30	30.5	31	31.5	32	32.5	33	33.5	34	34.5
Minimum Fire Damage	160	169.5	181	192.5	204	215.5	227	238.5	252	265.5
Maximum Fire Damage	185.5	196	208.5	221	233.5	246	258.5	271	285.5	300

Stat	Level
Stat	31	32	33	34	35	36	37	38	39	40
Mana Cost	35	35.5	36	36.5	37	37.5	38	38.5	39	39.5
Minimum Fire Damage	279	292.5	306	319.5	333	346.5	360	373.5	387	400.5
Maximum Fire Damage	314.5	329	343.5	358	372.5	387	401.5	416	430.5	445

Stat	Level
Stat	41	42	43	44	45	46	47	48	49	50
Mana Cost	40	40.5	41	41.5	42	42.5	43	43.5	44	44.5
Minimum Fire Damage	414	427.5	441	454.5	468	481.5	495	508.5	522	535.5
Maximum Fire Damage	459.5	474	488.5	503	517.5	532	546.5	561	575.5	590

Stat	Level
Stat	51	52	53	54	55	56	57	58	59	60
Mana Cost	45	45.5	46	46.5	47	47.5	48	48.5	49	49.5
Minimum Fire Damage	549	562.5	576	589.5	603	616.5	630	643.5	657	670.5
Maximum Fire Damage	604.5	619	633.5	648	662.5	677	691.5	706	720.5	735

Reference

Fire Spells (Arreat Summit)
Librarian, Sympathy for the Devil Minions & Pets (FAQtoids)