Description^{?}  

3% chance per rank a slain creature will add zero kills to its Slayer assignment; 2% chance per rank it will add 2 kills.  
Release date  25 January 2016 (Update) 
Gizmo type  Weapon, Armour 
Maximum rank  5 
Trophytaker's is an Invention perk that has a chance for each kill on a Slayer assignment to count as either 2 kills or zero. The perk only affects assignments given by Slayer Masters and not normal or Special slayer contracts obtained from Markus outside of the Slayer Tower. It can be created in weapon and armour gizmos.
On average, trophytaker's works out to be a net increase in assignment length. If however you do fewer kills than assigned you will receive fewer slayer points for completing the task.
Rank  No KC  Double KC  Avg change 

0  0%  0%  ±0.00% 
1  3%  2%  +1.01% 
2  6%  4%  +2.04% 
3  9%  6%  +3.09% 
4  12%  8%  +4.17% 
5  15%  10%  +5.26% 
When the perk doubles kill count, it prompts the message "Trophytaker's perk: this kill has added double to your Slayer task tally." When a kill does not count, it prompts, "Trophytaker's perk: this kill did not contribute to your Slayer task tally."
Calculation
The effect of trophytaker's on task length can be summarised as follows (where $ R $ is the rank of the perk):
 Chance of a kill counting 0: $ R \times 3% $
 Chance of a kill counting 2: $ R \times 2% $
 Chance of a kill counting 1 (as normal): $ 100%  R \times 5% $
This can be combined to give the gross effect of the perk:
$ 100%  R \times 3% + R \times 2% = 100%  R \times 1% $
i.e. a 1% per rank task length increase. For example, with rank 5 trophytaker's, a task of 100 will, on average, have 5 remaining after 100 kills.
However, trophytaker's will also work on the additional kills generated by the perk  from the example above, the 5 remaining on the task will also get a 5% increase, and so on. This means the net effect calculation is infinite, and best represented as a series:
$ Increase = \sum_{n=1}^\infty (0.01R)^n $
This converges when $ R<100 $, which is true for all of our possible values. If you substitute in the possible rank values (1–5), you will get the results in the above table.
Sources
Material  Rarity  Perk ranks with X materials  

1  2  3  4  5  
Crystal parts  Common  0  1  1–2  1–3  1–4  
Spiked parts  Common  0  1  1–2  1–3  1–4  
Powerful components  Uncommon  1  1–2  1–3  1–5  1–5  
Variable components  Uncommon  1  1–2  1–3  1–5  1–5 
Suggested gizmos
Gizmo layout  Possible perks  

 

Trivia
 Even though the patch notes on 22 February 2016 implied that this perk works with Slayer contracts, this is not the case.
Abilities  
Damage/Accuracy modifying 

Damage reduction 

Adrenaline 

Prayer 

Additional items 

Experience 

Skilling 

Miscellaneous 
