I accidentally placed this over on the general 40k topics forum - I've copied it here in the correct location!
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Basic Deathmark units need to be taken in sets of 2 to be effective. Mark 2 targets and both units will wound on a 2+ vs. each other's targets. Great for board control.
To make Deathmarks useful at long range add a Cryptek - Harbinger of Transmogrification with a Harp of Dissonance (infinite range). Remove the armour from a Grey Knight Baby carrier anywhere on the table. Fixes the 24" range issue! Park them in-cover on your side of the board and plink away at uber-units across the board. With 2 Deathmark units and 2 Harps you can cover most of the board and abuse each others "Marked for Death" targets.
For Deathmark Deep Strike abuse - a Harbinger of Despair (finally - a use for the oddball staff!). Strength 8, AP1 Template - but with the "Marked for Death" rule they wound on a 2+ instead of the goofy leadership roll (usually a 3+ or 4+). Deep strike in and wipe-out almost any unit in the game in one shot. Ignores FNP and 2+ saves. Not bad for a 125 point squad (including Cryptek).