Quite an esoteric theme for this one, although it is readily flagged and accessible thanks to the preamble, “The beasts are 8″ (i.e. 8D). I thought it strange that the creatures in question were referred to as “beasts” but have just noticed, on starting this entry that it’s a play on today’s puzzle number (xy,666). I shall be looking to litter my conversations tomorrow with the answer to 13A, as my newly-acquired “phrase of the week”.

Across
1 SET PIECE - “sett peace”. I was only aware of the definition in a footballing context, not a firework one.
5 A,P,PIUS - as in he of the Appian Way. I think this clue is about as close as you can get to double-duty wordplay without actually overdoing the brinkmanship. (How many non-Pontiffs do you know named Pius?).
9 SPEC,IOUS
10 REGINA - (regain)*. “State prosecutor”, as in Regina vs …., is a neat touch.
13 BR(AND,N)AZI(-l) - And I thought that the last surviving Nazi was Seinfeld’s Soup Nazi.
14 VOYEUR - (you Rev)*, with “Turner” being the typically Cinephiliac anagrind.
21 COCKER - thematic
23 IN(FLUE)N,ZA
27 A,LIE,NATE (a net)* - Took we longer to crack than it ought ot have.
29 PS(ALTER)Y - “Instrument of change” is a prime example of a seamless lift/separate device.

Down
1 SUS,SEX - thematic
2 TRE(BIZ,O)ND - Unknown to me but guessable from wordplay alone. Turns out that its modern day name is Trabzon (fellow footy nerds may recognise the Turkish team Trabzonspor).
3,25A - IRISH WATER - thematic
4 CLUMBER - thematic
5 PREADAMIC - (hidden)
8 SPANIELS - (painless)*. The theme en clair.
15 ELOQUENCE - “New Queen Cole” being the nursery rhyme monarch’s latest squeeze, one presumes.
20,21 KING CHARLES - thematic
22 GRE(ED)Y - “sombrely clothed” = in grey.
24 FIELD - thematic