A signed Daijo Fujiwara Sadakuni and dated Keicho 10 (1605) was designated at the sixtieth session; before it the published sources knew only a Keicho 14 (1609) as his dated work, and they read his Keicho- activity from it. He was a smith of the Shimosaka school, and the places him as 「康継と最も密接な関係にあった下坂一門の刀工」 (the smith of the Shimosaka group in the closest relationship with Yasutsugu). That closeness was once read too literally. -period sword books and registers scarcely record him, and the few that do treat him as the same man as Yasutsugu, helped by the likeness of their Daijo signatures; today the two are understood as distinct smiths. He signs Daijo Fujiwara Sadakuni, often with -ju on the reverse. Honma's inventory of the Imperial collection records a youthful signature, 「越前国下坂貞国」. A small signed Daijo Fujiwara Shimosaka with -ju is judged his early work, since in the first Keicho years three smiths, the later Yasutsugu, Sadakuni and Kanenori, signed Shimosaka with the Daijo title.
His hand is above all a hand. The published sources write that he 「直刃を得意として乱刃の作は殆んどなく」 (excelled at and made almost no works in ), and that his work is 「直刃調のものが多く、僅かにほつれ、小足が入り、地刃共に沸も穏やかな感がある」 (mostly -toned, slightly frayed, with , the of and alike giving a gentle impression). Faint and small mix into the line, the is small and quiet, and the sinks rather than glows, with thin and an occasional . The runs straight to a small with , at times drawn down long or a touch pointed. A in Honma's inventory widens the manner into a tone undulating shallowly with , the subdued throughout. Honma's entry sums him up the same way: nearly all his work is , with on some ; are many, few, and rare.
The is where the published sources separate him from Yasutsugu. Beside the master's work, they write, his shows 「黒ずんだ杢肌などは少なく、概ね鍛えがつみ」 (little of the blackish , the forging generally packed). It is an that runs and flows, mixed with , the leaning to , the grain standing somewhat under well-applied . One description states plainly that 「板目が流れているものが常で」 (the usually flows). On his better blades the sources find the tighter and more refined than ordinary work, and they name that refinement as the point of interest of this smith. On the Keicho 10 the steel takes on a dark cast, yet its precision still shows that hallmark.
A few break the rule, and the published sources treat them as signs of his range. One, a wide, extended kata- , has a low mixed with over a refined , with , and fine and . It is read as a work in which he 「古作の貞宗や信国に範をとった」 (took the old Sadamune and as his model), a piece of subdued flavour. Another departs entirely from his norm. On a wide, extended he tempers a broad mixed with , round-headed and pointed teeth, the long and frequent, the thick with coarse grains, and running strongly. The sources liken it to 「一派の中でも最も丁子の目立つ日向大掾貞次」 ( Daijo Sadatsugu, whose stands out above the rest of the school), and find it remarkable that a smith whose rule is a gentle should temper so brilliant a . Both blades are called 「貞国会心の一口」 (a piece into which Sadakuni put his whole heart), and the second 「同作中抜群の出来映え」 (outstanding among his works). The rest of the group stays nearer home, with shallow and on the .
Carving is the third strand of his work. His blades carry deep, strong -bori, 「越前彫独得のもの」 (of the kind peculiar to carving): a standing on a rock within a , a , and . Of the rock-borne on his early Shimosaka-signed the writes that it is a typical carving and very probably his own hand. Honma's inventory adds a carved with crane and tortoise, an auspicious subject, and a plum branch. Honma asks whether Sadakuni's plum and bamboo stand at the source of -bori, finds some basis for the tradition that he first carved the blades of the first Yasutsugu, and supposes that the two fell out and that Sadakuni ended his life in . His teachers and pupils are not recorded.
Fujishiro rates him Jo , and eleven designated works stand on record, all of them signed: nine and the and recorded in Honma's inventory. He holds no designation above , and no provenance or holding institution is recorded against his blades. Several carry the full long signature in fine chisel strokes, and one bears a six-character without Fujiwara, which the published sources call unusual. One of the blades is a , a form they say survives from his hand now and then but seldom with an . For a collector he is a careful and individual name rather than a famous one. Blades by him come to market only from time to time, a or at level, and a signed and dated example is the piece to wait for.