Help Me Pick a Template

[Foeofthelance]

So I'm putting together a final dungeon run for my group, now thankfully toned down both in numbers and jerkish-noobishness. Its going to be a level 25 1-shot, and since its quite possibly the last I'm going to see of these guys due to life reasons, I want it to be memorable.

The plot is simple. Down an ice field is a dragon guarding a temple where great evil lies imprisoned. This temple has been consecrated to Orcus, and contains the darkest creatures ever to feed on death. There will be Runespired Dracoliches and Death Titans, Aspects of Orcus and even a guest appearance by Doresain, the Ghoul King and his court of Dread Wraiths.

But they are not the true threat, for they are not there as prisoners, but as wardens, for at the heart of the temple lies a Balor, one powerful enough to threaten Orcus himself.

What template would best fit this then? Something to make the Balor powerful enough to be a threat, but not so overwhelming that it turns into a TPK (at least not without some assistance from the players!) and something that doesn't clash with the Balor's style. The game is 4ED, and please include the source. Suggestions?

[SurlySeraph]

Slapping on the Paragon template is always easy; however, I'd tone it down considerably (skipping the ability score boosts and special qualities) to keep it from being overpowering. Pseudonatural is also an option for added memorable-ness. Either of these will make it very difficult, but still possible to kill.