With its requirements met (Str 18 · Dex 15 · Fai 13), Black Steel Twinblade reaches 432 attack rating at +25; the strongest affinity at those stats is Flame Art at 568.
Standard affinity at max upgrade, each weapon at its own requirements. A full bar is the class's strongest (Euporia, 563).
Moveset from the game files
Damage with Standard +25 at the stats above, before the enemy's defense; two-handed moves include two-handing's Strength bonus. MV (motion value) is the hit's share of attack rating (100 = all of it).
One-handed16players can parry them
Move
MV
Damage
Stance
Stamina
1st hit
R1 1Standard
100
432
5
15
0.50 s
R1 2Standard
101
437
5
15
0.43 s
R1 3Standard
102
441
5
15
0.40 s
R1 4Standard
103
445
5
15
0.40 s
R1 5Standard
110
476
10
15
0.53 s
R2Pierce
125
541
10
25.7
—
Charged R2Pierce
160
692
30
38.5
1.40 s
R2 2Pierce
125
541
10
25.7
—
Charged R2 2Pierce
160
692
30
38.5
1.50 s
Running R1Pierce
105
454
5
19.3
0.53 s
Running R2Standard/Pierce · 2 hits
50 + 115
714
10
25.7
0.40 s
Rolling R1Standard
90*
380
5
12.8
0.37 s
Backstep R1Pierce
90
389
5
12.8
0.47 s
Jumping R1Standard
107
463
7.5
12.8
0.50 s
Jumping R2Pierce
127
549
20
25.7
0.53 s
Guard counterStandard
125
541
30
25.7
0.70 s
Two-handed16players can parry them
Move
MV
Damage
Stance
Stamina
1st hit
R1 1Standard · 2 hits
50 + 103
682
5.5
17.1
0.40 s
R1 2Standard · 2 hits
50 + 104
686
5.5
17.1
0.40 s
R1 3Standard
115
512
6.5
17.1
0.63 s
R1 4Standard · 2 hits
50 + 106
695
5.5
21.4
0.37 s
R1 5Standard · 3 hits
50 + 50 + 100
891
11
21.4
0.30 s
R2Pierce
135
601
11
32.1
—
Charged R2Standard/Pierce · 4 hits
50 + 50 + 50 + 165
1,404
33
42.8
1.30 s
R2 2Standard/Pierce · 2 hits
50 + 130
802
11
32.1
—
Charged R2 2Standard/Pierce · 4 hits
50 + 50 + 50 + 165
1,404
33
42.8
1.33 s
Running R1Standard · 2 hits
50 + 110
713
5.5
21.4
0.43 s
Running R2Standard/Pierce · 4 hits
40 + 40 + 40 + 125
1,092
12
30
0.37 s
Rolling R1Standard · 2 hits
50 + 95
646
5.5
15
0.30 s
Backstep R1Pierce
105
468
6.5
15
0.47 s
Jumping R1Standard · 2 hits
50 + 112
722
8.3
15
0.10 s
Jumping R2Pierce
150
668
20
30
0.50 s
Guard counterStandard · 2 hits
50 + 130
802
33
30
0.60 s
Criticals2
Move
MV
Damage
Stance
Stamina
1st hit
BackstabPierce
230
995
0
19.3
0.47 s
RipostePierce
345
1,493
0
19.3
0.80 s
Skill: Spinning Slash2move names inferred
Move
MV
Damage
Stance
Stamina
1st hit
SkillStandard
105
454
14.5
21.4
0.60 s
Skill follow-upStandard · 2 hits
60 + 110
736
19
32.1
0.27 s
From the player's animation events, BehaviorParam_PC and AtkParam_Pc. First hit: seconds at normal speed from the start of the move's animation (uncharged R2s start from a released charge, so they show —). Criticals include the weapon's critical multiplier. * The elemental motion value differs from the physical one shown; the damage uses each.
By upgrade level (Standard)
Level
AR
Physical
Holy
Cost
+0
165
131
34
—
+1
175
139
36
Smithing Stone [1] × 2
+2
185
147
38
Smithing Stone [1] × 4
+3
195
155
40
Smithing Stone [1] × 6
+4
205
163
42
Smithing Stone [2] × 2
+5
216
171
45
Smithing Stone [2] × 4
+6
226
179
47
Smithing Stone [2] × 6
+7
237
188
49
Smithing Stone [3] × 2
+8
247
196
51
Smithing Stone [3] × 4
+9
257
204
53
Smithing Stone [3] × 6
+10
268
213
55
Smithing Stone [4] × 2
+11
278
221
57
Smithing Stone [4] × 4
+12
290
230
60
Smithing Stone [4] × 6
+13
300
238
62
Smithing Stone [5] × 2
+14
311
247
64
Smithing Stone [5] × 4
+15
322
256
66
Smithing Stone [5] × 6
+16
332
264
68
Smithing Stone [6] × 2
+17
343
273
70
Smithing Stone [6] × 4
+18
354
282
72
Smithing Stone [6] × 6
+19
366
291
75
Smithing Stone [7] × 2
+20
376
299
77
Smithing Stone [7] × 4
+21
387
308
79
Smithing Stone [7] × 6
+22
398
317
81
Smithing Stone [8] × 2
+23
409
326
83
Smithing Stone [8] × 4
+24
420
335
85
Smithing Stone [8] × 6
+25
432
344
88
Ancient Dragon Smithing Stone × 1
From +0 to +25: 12 Smithing Stone [1], 12 Smithing Stone [2], 12 Smithing Stone [3], 12 Smithing Stone [4], 12 Smithing Stone [5], 12 Smithing Stone [6], 12 Smithing Stone [7], 12 Smithing Stone [8], 1 Ancient Dragon Smithing Stone.
Where to find Black Steel Twinblade
Source not yet traced: the game files we read don't say yet how to get this item.