What about making the 2nd stage a combat/skill challenge encounter?
A series of glyphs and symbols could glow on the giant crystals surface which starts to crack and splinter releasing those trapped within. The PCs have to fight the escaping creatures while trying to reseal the crystal prison they have weakened.
To make it more exciting you could ask for an arcane or religion check (DC 25) to estimate that they have about a minute (10 rounds) before the crystal breaks down and releases its entire contents (which would be bad).
You could have 3 separate cracks that need fixing, two small and one big.
Small Crack (3 successes required)
Looking at the crystal you can see a thin crack about half an inch wide running about 2 foot in a rough zigzag. From somewhere deep inside the crack an ominous purple light glows and as you draw near to examine it you can hear the turbulent echo of thousands of whispers imploring you to release them.
50 gps worth of residuum and an Arcana or Religion check as a standard action
18 or less – you made it worse, (add 1 to the number of successes required)
19 to 24 – it’s harder than it looks, keep trying
25 to 30 – that’s definitely working, keep at it, (1 success)
31 or more – that got it, no problem, (2 successes)
Note: allow a +2 bonus assist with creative use of another skill (as a standard action) by another PC.
At the beginning of each round the crack is open 4 Splinter Spirits (minion) or 1 Splinter Soul (standard creature) emerge from the crack. On turn 1 start with 4 minions.
Big Crack (2 stage challenge; each requiring 3 successes)
Running horizontally you can see the obvious results of internal failure, a large crack which at points is a good 12 inches across and runs for 6 foot on the surface and penetrates deep inside the crystal glows with a pulsing green and purple light. As you approach you can hear the anguished cries of trapped creatures desperately calling you to free them, and peering into the crack you catch sight of movement within.
Stage 1 (3 successes required)
The depth of this crack will require some serious magical binding to repair, but once the deepest damage has been cleared it should become easier.
200 gps worth of residuum and an Arcana or Religion check as a standard action
For this section each additional 100 gps worth of residuum used in a skill check will grant a +1 bonus to the roll.
Each time a PC makes a skill check they take 15 psychic damage.
20 or less – you made it worse, (add 1 to the number of successes required)
21 to 26 – it’s harder than it looks, keep trying
27 to 32 – that’s definitely working, keep at it, (1 success)
33 or more – that got it, no problem, (2 successes)
At the beginning of each round this stage of the crack is open 4 Splinter Spirits (minion) or 1 Splinter Soul Slasher (standard creature) emerge from the crack these coalesce in any free square within 10 squares of the crystal. On turn 1 start with 1 Spirit Soul Slasher.
Just as the final success is being achieved there is pulse of energy and surge of Shard Spirits pour out into the room, washing through anyone near the crack;
Burst 3; attack +17 vs. Fortitude; 3d10+6 psychic damage
Stage 2 (3 successes required)
The crack is now looking easier to repair, if anything slightly easier than the other small cracks as long as you keep reinforcing your magic with additional residuum.
100 gps worth of residuum and an Arcana or Religion check as a standard action
For this section each additional 100 gps worth of residuum used in a skill check will grant a +1 bonus to the roll.
18 or less – you made it worse, (add 1 to the number of successes required)
19 to 24 – it’s harder than it looks, keep trying
25 to 30 – that’s definitely working, keep at it, (1 success)
31 or more – that got it, no problem, (2 successes)
At the beginning of each round the crack is open 4 Splinter Spirits (minion) or 1 Splinter Soul (standard creature) emerge from the crack. On turn 1 start with 4 minions.
When all the cracks are repaired the crystal glows with a green light that shifts through purple to red, then becomes opaque as the light fades. The crystal is now back in its dormant state. The remaining spirit creatures could either fight to the death or attempt to flee in order to survive and attempt to reactivate the crystal at a later time.
Pro’s of this idea
It’s a race against time with the group splitting into teams sealing the crystal and defending the casters while they work. Constant swarming enemies should keep the pressure up, but gets easier as the cracks are dealt with one by one.
You can tailor the amount of creatures emerging as necessary to maintain the drama and balance of the encounter as you go along if necessary.
Con’s of this idea
The skill challenge (as written) is highly dependant on Arcana and Religion and isn’t particularly diverse (essentially the same roll every turn). Also it assumes that the PCs carry residuum around with them (I assume they do).
General
You will need to check the skill DCs against your group, and how easy you want to make the encounter. Remember with standard successes they need 12 successes to seal the crystal, by my rough reckoning 2 PCs using liberal amounts of additional residuum on the big crack should be able to do it. 3 PC trained in arcane or religion should be fine. (as you said you have 7 people in the group my guess is that it should be ok, but you will know better than me!).
What do you think , is this a good idea or not?
A series of glyphs and symbols could glow on the giant crystals surface which starts to crack and splinter releasing those trapped within. The PCs have to fight the escaping creatures while trying to reseal the crystal prison they have weakened.
To make it more exciting you could ask for an arcane or religion check (DC 25) to estimate that they have about a minute (10 rounds) before the crystal breaks down and releases its entire contents (which would be bad).
You could have 3 separate cracks that need fixing, two small and one big.
Small Crack (3 successes required)
Looking at the crystal you can see a thin crack about half an inch wide running about 2 foot in a rough zigzag. From somewhere deep inside the crack an ominous purple light glows and as you draw near to examine it you can hear the turbulent echo of thousands of whispers imploring you to release them.
50 gps worth of residuum and an Arcana or Religion check as a standard action
18 or less – you made it worse, (add 1 to the number of successes required)
19 to 24 – it’s harder than it looks, keep trying
25 to 30 – that’s definitely working, keep at it, (1 success)
31 or more – that got it, no problem, (2 successes)
Note: allow a +2 bonus assist with creative use of another skill (as a standard action) by another PC.
At the beginning of each round the crack is open 4 Splinter Spirits (minion) or 1 Splinter Soul (standard creature) emerge from the crack. On turn 1 start with 4 minions.
Big Crack (2 stage challenge; each requiring 3 successes)
Running horizontally you can see the obvious results of internal failure, a large crack which at points is a good 12 inches across and runs for 6 foot on the surface and penetrates deep inside the crystal glows with a pulsing green and purple light. As you approach you can hear the anguished cries of trapped creatures desperately calling you to free them, and peering into the crack you catch sight of movement within.
Stage 1 (3 successes required)
The depth of this crack will require some serious magical binding to repair, but once the deepest damage has been cleared it should become easier.
200 gps worth of residuum and an Arcana or Religion check as a standard action
For this section each additional 100 gps worth of residuum used in a skill check will grant a +1 bonus to the roll.
Each time a PC makes a skill check they take 15 psychic damage.
20 or less – you made it worse, (add 1 to the number of successes required)
21 to 26 – it’s harder than it looks, keep trying
27 to 32 – that’s definitely working, keep at it, (1 success)
33 or more – that got it, no problem, (2 successes)
At the beginning of each round this stage of the crack is open 4 Splinter Spirits (minion) or 1 Splinter Soul Slasher (standard creature) emerge from the crack these coalesce in any free square within 10 squares of the crystal. On turn 1 start with 1 Spirit Soul Slasher.
Just as the final success is being achieved there is pulse of energy and surge of Shard Spirits pour out into the room, washing through anyone near the crack;
Burst 3; attack +17 vs. Fortitude; 3d10+6 psychic damage
Stage 2 (3 successes required)
The crack is now looking easier to repair, if anything slightly easier than the other small cracks as long as you keep reinforcing your magic with additional residuum.
100 gps worth of residuum and an Arcana or Religion check as a standard action
For this section each additional 100 gps worth of residuum used in a skill check will grant a +1 bonus to the roll.
18 or less – you made it worse, (add 1 to the number of successes required)
19 to 24 – it’s harder than it looks, keep trying
25 to 30 – that’s definitely working, keep at it, (1 success)
31 or more – that got it, no problem, (2 successes)
At the beginning of each round the crack is open 4 Splinter Spirits (minion) or 1 Splinter Soul (standard creature) emerge from the crack. On turn 1 start with 4 minions.
When all the cracks are repaired the crystal glows with a green light that shifts through purple to red, then becomes opaque as the light fades. The crystal is now back in its dormant state. The remaining spirit creatures could either fight to the death or attempt to flee in order to survive and attempt to reactivate the crystal at a later time.
Pro’s of this idea
It’s a race against time with the group splitting into teams sealing the crystal and defending the casters while they work. Constant swarming enemies should keep the pressure up, but gets easier as the cracks are dealt with one by one.
You can tailor the amount of creatures emerging as necessary to maintain the drama and balance of the encounter as you go along if necessary.
Con’s of this idea
The skill challenge (as written) is highly dependant on Arcana and Religion and isn’t particularly diverse (essentially the same roll every turn). Also it assumes that the PCs carry residuum around with them (I assume they do).
General
You will need to check the skill DCs against your group, and how easy you want to make the encounter. Remember with standard successes they need 12 successes to seal the crystal, by my rough reckoning 2 PCs using liberal amounts of additional residuum on the big crack should be able to do it. 3 PC trained in arcane or religion should be fine. (as you said you have 7 people in the group my guess is that it should be ok, but you will know better than me!).
What do you think , is this a good idea or not?
Ghostly Caress (Standard; at-will)
Hunger of the Ancients (Standard; at-will)
Psychic Torment (Standard; at-will) 
Panarii Champion (Standard; encounter) 
). The target is dominated and take 10 ongoing necrotic damage, the target must succeed on 2 saving throws to end the effect (it still only gets 1 save at the end of its turn).
