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Edit
I mis-understood the rules. Sending this post down as it's not productive for good mathhammer.

This helps Harlequin players too for the record, so I would recommend posting there.

I'm almost certain your math is way off.

If your opponent is hitting on 2+ you will affect them 69% of the time and reduce their damage output by 20% when you do so. You are reducing their damage on average by 14%.

If your opponent is hitting on 3+ you will affect them 44% of the time and reduce their damage output by 25% when you do so. You are reducing their damage on average by 11%.

4+ is affected 25% of the time and reduced by 33% when affected. With an average damage reduction of 6%.

5+ is affected 11% of the time and reduced by 50% when affected. With an average damage reduction of 6%.

6+ is affected 3% of the time and reduced by 100% when affected. With an average damage reduction of 3%.

The better the likelyhood of your opponent hitting you the more likely Locus of Trickery is to trigger, but at the same time the effect is lessened, but overall the likelyhood of it happening is so much lower against things which are having a hard time hitting that it does not matter.

You have a 31% chance that your highest roll on the 2d6 is a 6. That gives you a 5/6 chance of reducing the damage of units that hit on 2+ by 20%, 4/6 chance of reducing the damage of units that hit on 3+ by 25%, 3/6 chance of reducing the damage of units that hit on 4+ by 33%, 2/6 chance of reducing the damage of units that hit on 5+ by 50%, 1/6 chance of reducing the damage of units that hit on 6+ by 100% with a re-roll.

25% the highest roll is a 5. That gives you a 5/6 chance of reducing the damage of units that hit on 2+ by 20%, 4/6 chance of reducing the damage of units that hit on 3+ by 25%, 3/6 chance of reducing the damage of units that hit on 4+ by 33%, 2/6 chance of reducing the damage of units that hit on 5+ by 50% and means you cannot reduce the damage dealt by units that hit on 6+ with a re-roll.

19% the highest roll is a 4. That gives you a 5/6 chance of reducing the damage of units that hit on 2+ by 20%, 4/6 chance of reducing the damage of units that hit on 3+ by 25%, 3/6 chance of reducing the damage of units that hit on 4+ by 33% and means you cannot reduce the damage dealt by units that hit on 5+ with a re-roll.

14% the highest roll is 3. That gives you a 5/6 chance of reducing the damage of units that hit on 2+ by 20%, 4/6 chance of reducing the damage of units that hit on 3+ by 25% and means you cannot reduce the damage dealt by units that hit on 4+ with a re-roll.

8% the highest roll is 2. That gives you a 5/6 chance of reducing the damage of units that hit on 2+ by 20% and means you cannot reduce the damage dealt by units that hit on 3+ with a re-roll.

3% the highest roll is a 1 and means you cannot reduce the damage dealt with a re-roll.

The enemy unit attacking you having re-rolls have no impact on the math of this ability, you are still reducing the damage by the same amount based on what your opponent is hitting on, whether your opponent is hitting on 4+ because he is WS3+ with -1 or WS5+ with +1 doesn't matter.

I think what your mistake may have been is you assumed it blocks all hits above or below what you rolled, but it only blocks the hit rolls (after re-rolls before modifiers) of the actual lowest roll you made for Locus of Trickery.

 vict0988 wrote:
I
3% the highest roll is a 1 and means you cannot reduce the damage dealt with a re-roll.

I think what your mistake may have been is you assumed it blocks all hits above or below what you rolled, but it only blocks the hit rolls (after re-rolls before modifiers) of the actual lowest roll you made for Locus of Trickery.

Thanks for you input. Peer review is always good

Lets look at the wording here.

Roll 2 dice at the start of each Fight phase and discard the highest result. Until the end of that phase, each time your opponent targets a TZEENTCH DAEMON unit within 6" of a friendly model with the Locus of Trickery and makes a hit roll that, after re-rolls but before modifiers, matches your remaining dice result, that hit roll fails

In your notes, you mention the highest roll. From what I read in the rule, we don't care about the highest roll (other than to throw the dice into a bucket). It's the lowest roll that we care about.

We take all the to-hit rolls of the attacking unit, and if they match your remaining dice then the attack will miss. Lets say you rolled a 3 and a 5 with your loci. You were attacked by a WS 2+ unit who rolled a [1, 2, 3, 4, 5] to hit. You would throw away the one automatically. The loci would reduce the number of hits from 4 in this case to 3.

If that same attack was made by a WS 3+, the loci would actually have no effect given that dice roll. This is because the lowest dice was a 3, which equaled the WS of the attacking unit. If you rolled a [4, 6] with your loci, then only 2 attacks from the WS 3+ unit would have hit instead of the 3 attacks.

If I am reading the rule for the loci wrong, please let me know.

I think you misunderstand it, yeah.

If you roll, for the Locus, a 6 and a 6, then any 6's are automatically discarded. They miss. However, if the opponent has WS of, say, 4+, then 4's and 5's still hit.

 labmouse42 wrote:

 vict0988 wrote:
I
3% the highest roll is a 1 and means you cannot reduce the damage dealt with a re-roll.

I think what your mistake may have been is you assumed it blocks all hits above or below what you rolled, but it only blocks the hit rolls (after re-rolls before modifiers) of the actual lowest roll you made for Locus of Trickery.

Thanks for you input. Peer review is always good

Lets look at the wording here.

Roll 2 dice at the start of each Fight phase and discard the highest result. Until the end of that phase, each time your opponent targets a TZEENTCH DAEMON unit within 6" of a friendly model with the Locus of Trickery and makes a hit roll that, after re-rolls but before modifiers, matches your remaining dice result, that hit roll fails

In your notes, you mention the highest roll. From what I read in the rule, we don't care about the highest roll (other than to throw the dice into a bucket). It's the lowest roll that we care about.

We take all the to-hit rolls of the attacking unit, and if they match your remaining dice then the attack will miss. Lets say you rolled a 3 and a 5 with your loci. You were attacked by a WS 2+ unit who rolled a [1, 2, 3, 4, 5] to hit. You would throw away the one automatically. The loci would reduce the number of hits from 4 in this case to 3.

If that same attack was made by a WS 3+, the loci would actually have no effect given that dice roll. This is because the lowest dice was a 3, which equaled the WS of the attacking unit. If you rolled a [4, 6] with your loci, then only 2 attacks from the WS 3+ unit would have hit instead of the 3 attacks.

If I am reading the rule for the loci wrong, please let me know.

The highest roll matters when you are considering re-rolling. Let's say you get one 6 and one 1, now the 1 is going to take effect and you will have your opponent drop all 1s. If you re-roll it, then because the 6 is the highest roll, the other dice will automatically be the lowest roll and will therefore be the important roll. Does that make sense?

Another way of thinking of it, is if you have rolled a 2+ and a 1 and you want to drop the damage of units that are hitting on 2s, you have a 5/6 chance of doing so by re-rolling the 1 since if you roll anything but a 1, you will negate 20% of the hit rolls of units that hit on 2s.

If you rolled a 3+ and a 1/2 and you want to affect units that are hitting on 3s, you have a 4/6 chance of doing so by re-rolling the 1/2 you rolled.

If you don't think reducing the damage of melee attackers by 20% on average is good, then this ability is horrible. I'm pretty sure it applies to some Heretic Astartes units though, so while the anti-melee might sound dumb on your Pink Horror, compared to re-rolling failed charge rolls, having a Tzeentch Daemon Detachment backing up your Daemon Engines and/or Possessed might actually be the best choice, although that fight-twice Strat is huge if you are bringing a 20-man Possessed unit, but if you did something more MSU-esque then Tzeentch is probably your best bet IMO.

But the odds get worse as WS of your enemy decreases.

 labmouse42 wrote:

2.78% of the time, your opponent's unit won't be able to hit you.

Based on what? For that to happen not only you need to roll double 6 but also opponent need to have WS6+. If he has WS2+ and you roll double 6 then that means 6's are discarded but 2, 3, 4 and 5 still hit.