Concluding the Expulsion boxset’s adventures, the final instalment takes the TARDIS trio into the realm of Norse mythology.  

Shieldmaiden Hervor braves the ghosts of her ancestors to claim her birthright, the sword Tyrfing. Made by the Duergar, the Dwarf kings of the below, the blade is cursed: once drawn, it must kill. Even the Doctor cannot resist the power of the sword, and to the horror of Peri and Turlough, he is forced to take a life…

Big Finish Synopsis

A tale of cursed swords, mind criminals and family legacies, The Curse of the Duergar starts on strong footing. Having their scheduled TARDIS maintenance interrupted by a sword-wielding warrior named Hervor, the adventure’s first part sees the time-travelling team embark on a dangerous quest on an island filled with perils both natural and unnatural. Travelling across the island facing numerous hazards strikes a good balance for the story. The dispersal of action sequences mixed alongside more talkative sections ensures that, for the first half at least, the story keeps an ever-progressing pace. It also manages to situate Hervor’s prowess as a warrior and get The Doctor and Turlough some one-on-one time, which the boxset has sorely needed.

As the adventure progresses into its second half, however, these once strong aspects aren’t nearly so sturdy. Moving away from the mythological to more alien antics proves to do this story a bit of a disservice. The Duergar, a race of alien mind criminals, slows the pace of the story down considerably. Their abilities to psychologically attack their victims lead to several dialogue-heavy scenes and ones where characters barely move from a single room. Combined, they kill any momentum the story has as well as my engagement with the material.

Unfortunately, the issues with this story don’t merely stem from its shaky pacing. Throughout the adventure, it becomes increasingly evident that there are opportunities not being taken and ideas that don’t quite come to fruition. One of these moments comes when The Doctor and Turlough are separated from the rest of the cast. Given the distrust and tentative ground they’ve had together over the boxset, this might’ve been a good way to capitalise on some much-needed development between them. However, their time together passes without incident or interesting heart-to-heart. On the ideas side, one aspect that was explored was the 6^th Doctor briefly changing into the 5^th personality-wise. While this might’ve been more noticeable earlier in his incarnation, by this point, his sharp edges have been worn down, and there isn’t enough going on in the story to be able to distinguish between them as the writer seems to want. As a result, this plot point comes across as confusing more than anything else, as, without Peri mentioning the difference, it’s almost impossible to tell if anything’s changed.