This is a Killer-X. There is a disjoint 8(3) cage at R4C1.
Prelims
a) R12C4 = {15/24}
b) R1C56 = {18/27/36/45}, no 9
c) R2C56 = {39/48/57}, no 1,2,6
d) R23C8 = {29/38/47/56}, no 1
e) R45C2 = {39/48/57}, no 1,2,6
f) R6C34 = {14/23}
g) R78C3 = {49/58/67}, no 1,2,3
h) R89C4 = {18/27/36/45}, no 9
i) R89C9 = {49/58/67}, no 1,2,3
j) R9C23 = {19/28/37/46}, no 5
k) 8(3) disjoint cage at R4C1 = {125/134}
Steps resulting from Prelims
1a. R1C56 = {18/27/36} (cannot be {45} which clashes with R12C4), no 4,5
1b. R89C4 = {18/27/36} (cannot be {45} which clashes with R12C4), no 4,5
2. 45 rule on N1 1 innie R3C3 = 1, placed for D\, clean-up: no 4 in R6C4, no 9 in R9C2
2a. 45 rule on C3 2 outies R36C4 = 12 = [93] -> R6C3 = 2 (cage sum), 3 placed for D/, clean-up: no 3 in R2C56, no 2 in R2C8, no 8 in R3C8, no 6 in R89C4, no 8 in R9C2
2b. R3C34 = [19] = 10 -> R45C3 = 14 = {68}, locked for C3 and N4, clean-up: no 4 in R45C2, no 5,7 in R78C3, no 2,4 in R9C2
2c. Naked pair {49} in R78C3, locked for C3 and N7, clean-up: no 1,6 in R9C2
2d. Naked pair {37} in R9C23, locked for R9 and N7, clean-up: no 2 in R8C4 no 6 in R8C9
2e. Killer pair 1,2 in R12C4 and R89C4, locked for C4
2f. Killer pair 4,5 in R12C4 and R2C56, locked for N2
2g. Killer pair 3,7 in R45C2 and R9C2, locked for C2
2h. 5 in C3 only in R12C3, locked for N1
2i. 5 in C3 only in R12C3 = {35/57} = 8,12 -> R1C12 = 10,14 = {28/46/68}, no 3,7,9
2j. 5 in R3 only in R3C789, locked for N3, clean-up: no 6 in R3C8
2k. 9 in N1 only in R2C12, locked for R2, clean-up: no 2 in R3C8
2l. 3,6 in N2 only in R13C56, CPE no 3,6 in R1C78
2m. 6 in C4 only in R457C4, CPE no 6 in R56C5
2n. 3 on D\ only in R7C7 + R8C8, locked for N9
2o. 3 in R7C7 + R8C8, CPE no 3 in R5C8
2p. 45 rule on N2 2 remaining innies R3C56 = 9 = {27/36}, no 8
2q. 45 rule on N4 4(2+2) outies R3C56 + R4C89 = 18, R3C56 = 9 -> R4C89 = 9 = {18/27/36/45}, no 9 in R4C89
2r. 45 rule on C9 2 outies R46C8 = 10 = [19/28/37]/{46}, no 5,7,8 in R4C8, no 1,5 in R6C8, clean-up: no 1,2,4 in R4C9
2s. 3 in R1 only in 22(4) cage at R1C1 = [8635] or in R1C56 = {36}, 6 locked for R1 (locking cages)
3. 8(3) disjoint cage at R4C1 = {125/134} = {15}2/[34]1, no 4 in R4C1, no 5 in R7C2
3a. Killer pair 3,5 in 8(3) disjoint cage and R45C2, locked for N4
3b. 45 rule on N7 2 innies R7C12 = 7 = [52/61]
3c. Combined half cage R4C1 + R6C2 + R7C12 = [1552/3461] (cannot be [5152]), no 5 in R4C1, no 1 in R6C2
3d. 1 in N4 only in R456C1, locked for C1
3e. 5 in N4 only in R456C2, locked for C2
3f. 15(3) cage at R8C1 = {168/258}
3g. 1 of {168} must be in R8C2 -> no 6 in R8C2
3h. 6 in N7 only in R789C1, locked for C1, clean-up: no 4 in R1C2 (step 2i)
4. 45 rule on R89 2 outies R7C38 = 1 innie R8C6 + 11
4a. Min R7C38 = 12 -> no 1,2 in R7C8
4b. Max R7C38 = 17 -> max R8C6 = 6
5. 45 rule on R1234 2 outies R5C34 = 2 innies R4C12 + 6, R4C12 are both odd -> R5C3 is even -> R5C4 must be even = {468}
5a. Max R5C34 = 14 -> max R4C12 = 8, no 9 in R4C2, clean-up: no 3 in R5C2
5b. 3 in N4 only in R4C12, locked for R4, clean-up: no 6 in R4C89 (step 2q), no 4,7 in R6C8 (step 2r)
[I made a careless omission in my original step 5 so this is my new key step. Probably more complicated than the way that Ed solved this Assassin (this comment was made before I went through his WT); this step was triggered by thinking of a way to eliminate my omission, I actually realised this step while watching a DVD.]
6. Consider placement for 4 in N2
4 in R12C4, locked for C4
or in R2C56 = {48}, locked for R2 => R23C8 = [65/74] => R46C8 (step 2r) = [19/28] (cannot be [46] which clashes with R23C8) => 4 in R4 in R4C4567, locked for 28(5) cage at R4C4 => no 4 in R5C4
-> no 4 in R5C4
[Now things are fairly straightforward]
6a. Naked pair {68} in R5C34, locked for R5
6b. R5C34 = R4C12 + 6 (step 5), R5C34 = {68} = 14 -> R4C12 = 8 contains 3 (step 5b) = [35], R5C2 = 7 (cage sum), R6C2 = 4, R7C2 = 1 (cage sum), clean-up: no 4 in R4C8 (step 2q), no 6 in R6C8 (step 2r)
6c. Naked pair {19} in R56C1, locked for C1 -> R7C1 = 6 (cage sum)
6d. R2C2 = 9 (hidden single in N1), placed for D\, clean-up: no 4 in R8C9
6e. R9C23 = [37], R12C3 = {35} = 8 -> R1C12 = 14 = [86], 8 placed for D\, R3C2 = 2, R8C2 = 8, placed for D/, clean-up: no 1,3 in R1C56, no 3 in R3C8, no 5 in R89C9, no 1 in R9C4
6f. Naked pair {27} in R1C56, locked for R1 and N2, clean-up: no 4 in R12C4, no 5 in R2C56
6g. Naked triple {149} in R1C789, locked for R1 and N3 -> R12C4 = [51], R8C4 = 7 -> R9C4 = 2, R8C9 = 9 -> R9C9 = 4, placed for D\, R1C9 = 1, placed for D/, R8C3 = 4, R7C3 = 9, placed for D/, R4C4 = 6, placed for D\, R9C1 = 5, placed for D/, R5C5 = 2, placed for D\, clean-up: no 7 in R23C8
6h. R23C8 = [65], R3C7 = 7, placed for D/
6i. R4C8 = 2 -> R4C9 = 7 (step 2q), R6C8 = 8 (step 2r)
6j. Naked pair {36} in R3C56, locked for R3 -> R23C9 = [38], R567C9 = [562]
6k. R8C8 = 3, R7C7 = 5, placed for D\
6l. R6C5 = 5 (hidden single in N5)
and the rest is naked singles, without using the diagonals.