Instruments of chaos appear to be a rather good idea on paper. For a very modest points value investment, it is possible to turn a drawn close combat round in to a victory. Are they worthwhile? If so, then on which daemons?
As long term readers will know, I've been playing daemons since the codex came out. In that time, I've been experimenting with various army lists (fully mixed daemons right down to mono-chaos power lists).
What have I discovered? Well, I've come to the conclusion that instruments of chaos are only to be used with one type of daemon: plaguebearers. Let me explain.
First up, bloodletters of Khorne are superb close combat exponents. So long as they don't get shot up too badly on their way in to close combat, they're usually going to win. And they're going to win by a healthy and large margin thanks to all of those hell blades in unison with their furious charge. Bloodletters generally win big, or not at all. They hardly ever draw. Because of that, I find that instruments are a waste of points on bloodletters.
Greater daemons, daemon princes and bloodcrushers of Khorne are in a similar category - they shouldn't need instruments.
Daemonettes of Slaanesh are not quite as good as bloodletters. Yet, with their rending ability and sheer number of high initiative attacks on the charge, they are also likely to win big on close combats; or not at all. They just don't tend to draw combats much - just like bloodletters, but to a lesser extent. Hence, I find daemonettes shouldn't be taking instruments either. By extension, seekers of Slaanesh are also in the same category.
Horrors of Tzeentch are not built for close combat. They are shooty combined with a reasonably high invulnerable save. But their low toughness, low initiative and low number of attacks each (cf. daemonettes who have 3 base attacks, very high initiative, but low toughness) means that horrors are bound to lose close combats most of the time. Even imperial guard will find it okay to take horrors on in close combat. They simply lose big time unless facing isolated troops or small numbers of guard or tau. Therefore, they're unlikely to draw - they mostly lose in close combat.
That brings me to plaguebearers of Nurgle. Now, these are the daemons that can really exploit those instruments. Why is that? Well, plaguebearers have only one attack (base), but they have an incredibly good toughness for troops, coupled with the feel no pain special rule and poisoned attacks. These factors combined mean that they can often dish out as good as they get in close combat. Indeed, I've used them in battles to tie up everything ranging from dreadnoughts (which they cannot hurt!) to tyranid carnifexes for three or more turns. In numbers they can hold out against many other units in the game and, with a little luck, be deadly. But if they've not charged and have got in to their usual "tar pit" situation with an opponent, they can often cause tied combats. This is exactly where instruments come in to their own.
There's one other case that should be mentioned: that of an Epidemius mono-Nurgle army list. I haven't quite made up my mind about this situation. The thing is, plaguebearers steadily (hopefully rapidly!) become uber-troops with Epidemius present. They should, in theory, rapidly outgrow the need for instruments. Hence instruments are only useful in the first couple of turns and/or if Epidemius fails to make an early appearance and the plagebearers accidentally get in to an early combat.
