Augh, haven't been made to feel these sorts of feelings from a book in a long time...
What I liked also about this book is that Harmony's involvement is justified beyond, "hey, let's do this for some fan-service!" There is some crazy stuff going on that potentially involves another Shard... or some hitherto unknown Cosmere denizen of doom! In either event, I'm afraid this is point more towards the main conflict of Mistborn Trilogy Two (the Modern Era one), which is also pretty neat, since I was wondering how you could go beyond the main conflict in the Era 1 trilogy.
Also, it seems that the spikes in the perverted subterranean humans were also the same. On page 328:
And for comparison, from the epilogue: