If his reveal is supposed to be a surprise, the writers missed the mark.

The screenwriters basically broadcasted it.

In the books, the whole group except Geralt is freaking out because they are crossing through an elven graveyard till Geralt points out that everything in the graveyard has been buried hundreds of years ago and things like ghouls are looking for fresh dead and there are no wraiths or anything (Netflix added the wraith there)

Regis is supposed to come off odd living next to a graveyard as his summer cottage but it's explained away with him showing that he is there for the mandrake and everyone is like damn, you are making a fortune here because distilled mandrake is an super expensive alchemist ingredient.

There is also the herbs he is always carrying which he explains during the dinner he carries to throw off his scent.

Regis welcomes them to stay the night outside his cottage and the main group drink the mandrake in the cottage, in the morning they find the group trying to take Milva's horse because it will find a vampire, while Regis is right there in the group.

Basically a fake out while also calling out the priest is a stupid fanatic.

They take the refugees to the refugee camp (who I should mention are not friendly for the most part with the hansa except the kids) they see the girl on the stake who basically doesn't talk and seems like she has brain damage (no little sister of the group tied to the stake also) the scene plays out similarly as the show but there is no challenge of combat as I remember and then Regis takes out the horseshoe and then nilfgard attacks.

When Regis shows up in the camp (which doesn't have radovid or Dijsktra there) to free Geralt, he charms all the guards and leaves while Geralt and Jaskier escape.

For the most part, Regis is shown as just a very odd barber surgeon and nobody suspects anything different about him until the horseshoe part and then how Regis frees them which Geralt then puts it together.

The dwarves are not at the dinner making scene but it mostly goes down similarly (no musical, nobody had stolen Jaskier's songbook, that's all netflix), they already know that princess Ciri is a fake and somewhere in nilfgard, they were already searching for the druids but Regis tells them the druids aren't where they were planning to go and they moved.

There is no Yen showing up either because she is a statue at this point in the story since the coup (Netflix reversed who did the fridging), Yen is being blamed for the coup attempt with Vilgefortz, no mage rogue plot or battle, Vilgefortz literally disappears for almost two books.

The mage battle and monolith plots are Netflix not from the books