I'm stealing AbdulAlhazred's idea, but am not giving XP for that pun.
I'm not going to depict a round, because that would require me to think too hard about PC builds. I will sketch an encounter (I'm not used to designing for 3 PCs, but will try) for Level 3 PCs. I think it is fairly hard but not impossible.
It will also be trope-heavy.
Setting: the stone walled but earth covered burial mounds of the temple elders. The entry way is open, because the evil cultists have opened it for their nefarious purposes. The PCs may have had to fight their way in already, dealing with the cult rabble (mostly minions, one leader-type) who were guarding the outside (and too cowardly to enter). It's possible that a survivor from that first assault fled down the tunnel - even if that was the leader-type, his/her cowardly nature means that, in the barrow, s/he is reduced to minion status.
The battle map in words: I'm thinking maybe 5 x 7 squares - or smaller, 4 x 6 squares. (I'm not used to 3 PCs!) Movement has to be a significant constraining factor, so it can't be too big. All the creatures are medium.
In the centre of the left (ie non-entry, non-rear) wall of the chamber is an open pit, 20' deep, where the body of a temple elder has been exhumed. This is 2 x 1 squares. On the side of the pit, opposite the entrance, is a pile of earth from the grave - 3 x 2 squares, difficult terrain (DC 10 Athletics to avoid this penalty). (I know my earth squares are fewer than my pit squares, but some of the earth is scattered elsewhere on the floor, but not deep enough to cause movement issues.)
The pit itself is welling with necrotic energy. Anyone who falls into it takes standard falling damage, plus 5 necrotic, 5 OG (leaving the pit ends this). At the start of the third round, this steps up to 10 necrotic and any current OG damage also steps up (and the necrotic energy becomes palpably more terrible) - this counts as a skill challenge failure. At the start of the fourth round, everyone in the room also takes 5 necrotic damage at the end of his/her turn - this counts as a skill challenge failure. At the start of the fifth round, a third failure is accrued.
Anyone in the pit also suffers a -2 to checks in the skill challenge. Climbing out of the pit is DC 13 Athletics.
Next to the entrance (on the side closer to the wight - see below - I think) is a ceremonial gong.
Creatures: Standing at the end of the pit is the (fallen, coup-leading) abbot who summoned the demon back when the sohei PC was a serving temple warrior, and who has raised the temple elder from death into undeath. I'm lazy, and already have my MM open, so I'm saying that this is a Human Mage but with Darkbolts (necrotic damage) instead of magic missile, Soul Drain (necrotic again, and attacks Fort, not Ref) instead of Dancing Lightning, and Miasma of Despair instead of Thunder Burst (attacks Will, not Fort, and does psychic damage). He also has necrotic resistance 5 from his Abyssal prayer beads. (This can be a level 4 or 5 neck item if taken by the PCs - but wearing these prayer beads causes a -5 penalty to checks to checks in the skill challenge.)
Tactically, this NPC will move into the difficult terrain in the corner behind the pit.
Between the entrance and the pit is a Deathlock Wight - the risen temple elder. These guys are perfect as they are (other than the standard damage fixes, and making the damage from Horrific Visage psychic, which I think they picked up in the PDF with the MV reworking of wights). It doesn't have any friends to reanimate, but that will have another use: it can use its Reanimate to power up the necrotic energy in the pit (if adjacent) or in the room (from round 4 on), undoing one success in the skill challenge.
Tactically, anyone who tries to go past it to the abbot gets pushed into the pit. It should fall back to a wall, so that it's blast can push into the pit. If the PCs try and keep away to jump the pit, it uses gravebolt to try and stop this, and if they make it to the other side there will still be a square of pit in front of them (and next to the abbot in the corner), so the wight can try and push them into that.
The skill challenge is Level 3, complexity 1 (E/M/H DC: 9/13/21). Each success delays the progression of the necrotic energy by one round (no stacking) or inflicts 1d10 damage on the wight. Failure causes the same damage to the PC (it may be necrotic or psychic, depending on context).
Possible actions in the skill challenge (off the top of my head) include throwing sanctified earth back into the pit (Athletics), using prayers or magic (Religion or Arcana), or striking the gong with the right tone (Religion with a +2 bonus the first time because it's cool, or maybe History - there's no music skill in 4e). Maybe other stuff I'm not thinking of (eg Intimidate to taunt the abbot). I think the gong could also be used to undo a failure (with a Hard check).
If the skill challenge fails then the corruption of the tomb is complete - the welling necrotic energy drives the PCs from the tomb (the combat is over, but everyone loses another HS). When they get out, they see any surviving cultists have fled too (they got more than they bargained for!), but the PCs still have to track them down/return home/head for the next mound to try to ensure it's protection/etc.
Budget and level: by my maths that's 175 x 2 + 150 = 500 XP, or a 4th level encounter for 3 PCs (a minion or two from cultists who flee inside is a rounding error for level). I think that it should be fairly challenging, given that if one PC falls into the pit that's fairly bad for action economy.
I'm not sure there's the right balance of incentives to skill vs incentives to fight - in my group I think the invoker/wizard would skill it up, because he's not a very good fighter, but a different group might just try and win the fight in (say) 3 rounds, and then try and win the skill challenge in the final round or two before it all explodes. I'll leave it to someone else to try and sketch a playthrough to see if it works out OK!
(That also took longer to write up than I thought it would - over 45 minutes by the time I looked up my monsters and tweaked some of the details.)