Here is another Epic level encounter for 6 level 22 PCs from my campaign. This was 3 Giant Dust Spiders and 1 Dust Spider Swarm.
This took place in a large dusty cavern with three rocky pillars (one spider hidden on each), with two exits one of which was blocked off with fallen rocks. The spiders were hidden and tried to gain surprise on the PCs (waiting until the PCs were in a good ambush spot), while the Spider Swarm entered in round two, pouring through the gaps between the rocks like a living waterfall.
Giant Dust Spider Level 22 Elite Skirmisher
Large Natural Beast XP 8,300
Initiative +23 Senses Perception +23, darkvision
HP 416; Bloodied 208; see Bloodied Withdrawal
AC 36; Fortitude 34, Reflex 37, Will 34
Vulnerable 10 cold
Saving Throws +2
Speed 7, climb 6 (spiderclimb)
Action Points 1
Leg Slam (Standard; at-will)
Reach 2; attack +27 vs. AC; 2d6+4 damage; on hit secondary attack
against the same target; attack +25 vs. Reflex; on hit target is knocked
prone
Bite (Standard; at-will) ♦ poison
Attack +27 vs. AC; 3d6+10 damage; on hit secondary attack +25 vs.
Fortitude; on hit target is slowed and takes 15 ongoing poison damage
(save ends both)
Lethal Injection (Standard; recharge 
) ♦ poison, prone target only
Prone target only, attack +25 vs. Reflex; on hit target is pulled into the
Spiders square and takes a secondary attack +27 vs. Fortitude; 4d12+7
poison damage; on hit target is restrained and takes 15 ongoing poison
damage (save ends both), aftereffect target takes 10 ongoing poison
damage (save ends)
Deadly Jump (Standard; recharge 
)
Dust Spider jumps (shifts) 5 squares and makes a Bite attack with a +1
bonus to hit
Choking Dust (Minor; recharge ) ♦ poison
Burst 3; enemies only; attack +25 vs. Fortitude; on hit target takes a -2
penalty to attack rolls and grants combat advantage to all enemies (save
ends both)
Bloodied Withdrawal (Immediate Reaction) ♦ when first bloodied
When first bloodied Dust Spider jumps (shifts) 5 squares and its Lethal
Injection power recharges
Threatening Reach
Dust Spider has threatening reach (2) with its Leg Slam attack
Alignment Unaligned
Skills Acrobatics +26, Athletics +21, Stealth +26
Str 20 (+16) Dex 30 (+21) Wis 24 (+18)
Con 24 (+18) Int 10 (+11) Cha 14 (+13)
Dust Spider Swarm Level 22 Brute
Gargantuan Natural Beast XP 4,150
Initiative +20 Senses Perception +10, darkvision
Carpet of Spiders aura 1; All none spider creatures adjacent to or sharing
a square with the swarm take a Hundred Bites attack at the start of their
turn; all non spider creatures sharing a square with the swarm take a -2
penalty to saving throws vs. ongoing poison damage
HP 254; Bloodied 127
AC 34; Fortitude 34, Reflex 36, Will 30
Immune see Swarm Characteristics;
Resist half damage from melee and ranged attacks;
Vulnerable 10 close and area attacks
Speed 5
Hundred Bites (Standard; at-will)
Attack +25 vs. AC; 2d6+8 damage; 3d6+8 damage against prone targets
Crawling Doom (Standard; at-will)
Targets all non spider creatures sharing a square with the swarm; attack
+23 vs. Fortitude; 3d6+8 damage (4d6+8 damage against prone targets);
on hit secondary attack +21 vs. Fortitude; on hit target is knocked prone
Dust Cloud (Standard; recharge ) ♦ poison
Attacks all creatures sharing a square with the swarm; attack +23 vs.
Fortitude; 2d10+8 poison damage; on hit target is blinded and dazed (save
ends both)
Mega Swarm
Dust Spider Swarm may move through and end its turn in other creatures
squares; the Swarm never provokes opportunity attacks and it may not
make opportunity attacks itself
Swarm Characteristics
Dust Spider Swarm cannot be blinded, stunned, dazed, immobilised,
slowed, knocked prone, put to sleep or knocked unconscious
Alignment Unaligned
Str 20 (+16) Dex 28 (+20) Wis 8 (+10)
Con 24 (+18) Int 6 (+9) Cha 3 (+7)
I thought this encounter worked pretty well. My PCs were able to keep the swarm at bay with area effect and pushing powers, they were also able to put down a zone of fire to shelter in that helped a lot. The spiders were able to skirmish around and move about which kept the encounter dynamic.
This took place in a large dusty cavern with three rocky pillars (one spider hidden on each), with two exits one of which was blocked off with fallen rocks. The spiders were hidden and tried to gain surprise on the PCs (waiting until the PCs were in a good ambush spot), while the Spider Swarm entered in round two, pouring through the gaps between the rocks like a living waterfall.
Giant Dust Spider Level 22 Elite Skirmisher
Large Natural Beast XP 8,300
Initiative +23 Senses Perception +23, darkvision
HP 416; Bloodied 208; see Bloodied Withdrawal
AC 36; Fortitude 34, Reflex 37, Will 34
Vulnerable 10 cold
Saving Throws +2
Speed 7, climb 6 (spiderclimb)
Action Points 1
Leg Slam (Standard; at-will)Reach 2; attack +27 vs. AC; 2d6+4 damage; on hit secondary attack
against the same target; attack +25 vs. Reflex; on hit target is knocked
prone
Bite (Standard; at-will) ♦ poisonAttack +27 vs. AC; 3d6+10 damage; on hit secondary attack +25 vs.
Fortitude; on hit target is slowed and takes 15 ongoing poison damage
(save ends both)
Lethal Injection (Standard; recharge ) ♦ poison, prone target onlyProne target only, attack +25 vs. Reflex; on hit target is pulled into the
Spiders square and takes a secondary attack +27 vs. Fortitude; 4d12+7
poison damage; on hit target is restrained and takes 15 ongoing poison
damage (save ends both), aftereffect target takes 10 ongoing poison
damage (save ends)
Deadly Jump (Standard; recharge )Dust Spider jumps (shifts) 5 squares and makes a Bite attack with a +1
bonus to hit
Choking Dust (Minor; recharge ) ♦ poisonBurst 3; enemies only; attack +25 vs. Fortitude; on hit target takes a -2
penalty to attack rolls and grants combat advantage to all enemies (save
ends both)
Bloodied Withdrawal (Immediate Reaction) ♦ when first bloodied
When first bloodied Dust Spider jumps (shifts) 5 squares and its Lethal
Injection power recharges
Threatening Reach
Dust Spider has threatening reach (2) with its Leg Slam attack
Alignment Unaligned
Skills Acrobatics +26, Athletics +21, Stealth +26
Str 20 (+16) Dex 30 (+21) Wis 24 (+18)
Con 24 (+18) Int 10 (+11) Cha 14 (+13)
Dust Spider Swarm Level 22 Brute
Gargantuan Natural Beast XP 4,150
Initiative +20 Senses Perception +10, darkvision
Carpet of Spiders aura 1; All none spider creatures adjacent to or sharing
a square with the swarm take a Hundred Bites attack at the start of their
turn; all non spider creatures sharing a square with the swarm take a -2
penalty to saving throws vs. ongoing poison damage
HP 254; Bloodied 127
AC 34; Fortitude 34, Reflex 36, Will 30
Immune see Swarm Characteristics;
Resist half damage from melee and ranged attacks;
Vulnerable 10 close and area attacks
Speed 5
Hundred Bites (Standard; at-will)Attack +25 vs. AC; 2d6+8 damage; 3d6+8 damage against prone targets
Crawling Doom (Standard; at-will)Targets all non spider creatures sharing a square with the swarm; attack
+23 vs. Fortitude; 3d6+8 damage (4d6+8 damage against prone targets);
on hit secondary attack +21 vs. Fortitude; on hit target is knocked prone
Dust Cloud (Standard; recharge ) ♦ poisonAttacks all creatures sharing a square with the swarm; attack +23 vs.
Fortitude; 2d10+8 poison damage; on hit target is blinded and dazed (save
ends both)
Mega Swarm
Dust Spider Swarm may move through and end its turn in other creatures
squares; the Swarm never provokes opportunity attacks and it may not
make opportunity attacks itself
Swarm Characteristics
Dust Spider Swarm cannot be blinded, stunned, dazed, immobilised,
slowed, knocked prone, put to sleep or knocked unconscious
Alignment Unaligned
Str 20 (+16) Dex 28 (+20) Wis 8 (+10)
Con 24 (+18) Int 6 (+9) Cha 3 (+7)
I thought this encounter worked pretty well. My PCs were able to keep the swarm at bay with area effect and pushing powers, they were also able to put down a zone of fire to shelter in that helped a lot. The spiders were able to skirmish around and move about which kept the encounter dynamic.
