This is the annotated solution to my A Song and Ice and Fire-themed cryptic crossword I Sent Letters About Cersei’s First Sexual Relationship.

Definitions are underlined, anagrams are starred, reversals are indicated by <.

Across

1 Heathen ironborn plundered plowed field. (7)
INFIDEL. I(ronborn)N FIELD*.

5 Behaviour of a cruel and terrifying person—I therefore left leaders of Sorrowful Men. (7)
OGREISM. (I ERGO)< S(orrowful) M(en).

9 Sellsword captain carried goods, having a change of heart towards early retirement. (5)
CAGGO, known as Corpsekiller, captain of the Windblown in A Dance of Dragons LX. CARGO (= carried goods) with central letter G changed to R(etirement). Not the best clue in the set.

10 Rev. Spooner escorted laggard to a guardsman sworn to House Lannister. (3,6)
RED LESTER, guardsman of Tywin Lannister, posted outside the Hand’s solar in A Storm of Swords LXXVII. Spoonerism of “led rester”. Arguably, the Spoonerism should be indicated by “According to Spooner” or something like that. The “a” is superfluous.

11 Short moments filled with endless lay for relatives. (6,4)
SECOND SONS, among the oldest of the free sellsword companies. SECONDS around SON(g).

12 Wants to sell daughter’s maidenhead and partially redeem maester. (4)
EMMA, old server at the Quill and Tankard tavern of Oldtown, mother of Rosey. Hidden in redeEM MAester. According to some schools, the definition ought to be “She wants to sell…”.

14 Ned heard two manoeuvres in despair. (11)
DOWNHEARTED. NEDHEARDTWO*.

17 Reward tenth sworn brother of the Night’s Watch. (6,5)
ENDREW TARTH, master-at-arms at Castle Black after Ser Alliser Thorne. REWARDTENTH*, with “sworn” (in the sense of “cursed” etc.) as the anagram indicator.

19 Leave out central promise to Targaryen leader. (4)
OMIT. prOMIse T(argaryen).

20 A few groats and pennies holding unit back before first ride for Ser Duncan at Ashford Meadow, say. (10)
CHALLENGER, which was the role of Ser Duncan in said tournament in The Hedge Knight. CHANGE around ELL< followed by R(ide).

23 Dwarf returned cup-shaped metal object to unknown artisan. (9)
IRONBELLY, master smith at King’s Landing charged with chain-construction by Lord Tyrion in A Clash of Kings XV. NORI (a dwarf in JRR Tolkien’s The Hobbit) reversed, followed by BELL and Y. I am very pleased with this clue: the unknown is well-hidden, the surface makes you think of an irrelevant situation (pointing towards the wrong dwarf and the wrong cup) at the purple wedding of A Storm of Swords.

24 Eyrie facing the French is situated in a sheltered position. (7)
NESTLES. NEST + LES.

25 Hedge knight’s coat of ice is surrounded by expression of surprise. (7)
LORIMER, known as the Belly, a fat hedge knight in the service of Leo Lefford, attended by Podrick Payne, see A Feast for Crows XIV. RIME in LOR

Down

1 I sent letters about Cersei’s first sexual relationship! (6)
INCEST. ISENT* about Cersei. Another clue I’m very happy with.

2 Initially festive, until Gwyneth and Cletus Yronwood’s banishment. (6)
FUCAGY. Acrostic in “F(estive)… Y(ronwood)”.

3 Return command to Eddard before soldiers become priests. (7,3)
DROWNED MEN, priests of the Ironborn. WORD< NED MEN. “Soldiers” for MEN is not really cryptic enough in my book.

4 Solar remodelled for knight of the Kingsguard. (5)
LORAS. SOLAR*.

5 Honor-clad, destroyed seat of a noble house in the Vale of Arryn. (3,6)
OLD ANCHOR, seat of House Melcolm . HONORCLAD*.

6 Noble house: A stag rampant. (4)
REED. DEER<.

7 At home, Tangletongue consumed close friend. (8)
INTIMATE (as a noun). IN TIM  ATE. Tim Tangletongue is a steward at Castle Black, defending the Wall in A Storm of Swords LXIX.

8 Sam mired in a mess, attending the Drowned God. (8)
MERMAIDS. SAMMIRED*. Again, some schools don’t like the grammar of the definition.

13 Atop Reek, he jerked Jaime’s junk. (10)
OATHKEEPER, Valyrian steel sword of Ser Jaime Lannister, discarded in A Storm of Swords LXXII by giving it to Brienne of Tarth. ATOPREEKHE*. Irresistible surface.

15 Fails to disclose how an area can be fortified? (9)
WITHHOLDS. An area can be fortified WITH HOLDS.

16 Maesters and the King’s Hand, for example, cover the face after short announcement. (8)
ADVISORS. AD VISORS

18 House Mallister’s heraldic beast, beheaded in the morning, catches sunlight. (6)
AGLEAM. (e)AGLE AM.

21 Staunch and kingly, after change of allegiance. (5)
LOYAL. ROYAL after changing directions (Left for Right).

22 Bard sounds talented. (4)
ABEL, bardic alter ego of Mance Rayder attending a feast in Winterfell. Homophone of ABLE.

Unclued

The unclued entries solve as PIETY, PRAYER, and DEVOTION, the three ships of the unfortunate Lord Sunglass seized by King Stannis for the attack on King’s Landing.