For SINGLE TARGET attacks, you will need the following TWO macros:
The MAIN MACRO…
/castsequence reset=0.2 0,0,Mind Blast
/castsequence reset=0.2 0,Shadow Word: Death
/castsequence [nochanneling] reset=target/3 Shadow Word: Pain, Vampiric Touch, Mind Flay, Mind Flay, Mind Flay
/cast !Power Infusion
/cast Shadowfiend
…and the MODIFIED DEVOURING PLAGUE MACRO for when you accumulate THREE SHADOW ORBS:
/castsequence [nochanneling] reset=6 Devouring Plague, Mind Flay, Mind Flay, null
/cast !Power Infusion
When attacking MULTIPLE TARGETS, you will need the following ONE macro:
/castsequence [nochanneling] reset=target/3 0,Mind Sear
/castsequence reset=0.2 Halo
/cast !Power Infusion
/cast Shadowfiend
Additionally, if you ever need to heal yourself while in Shadow specialization, here is a quality SELF HEAL MACRO:
/target <YourPlayerName>
/castsequence reset=0.2 0,0,Flash Heal
/castsequence reset=1.0 0,Prayer of Mending,Renew
/castsequence reset=0.2 Power Word: Shield
/cast !Power Infusion
/cast Shadowfiend
Also, depending on your talents and race, you may want to incorporate the following spell casts into any of the above-mentioned macros:
/cast Gift of the Naaru
* HEALING spell, for Draenei race
/cast Fade
** REDUCE DAMAGE spell, if you have chosen to use the Fade Glyph
/cast Spectral Guise
*** SURVIVABILITY spell, that will prevent your attacker from targeting you briefly
/cast Void Tendrils
**** SURVIVABILITY spell, that should allow you to distance yourself from melee attackers
/cast Vampiric Embrace
**** HEALING buff, that allows you and your allies to be healed for 50% of any single-target Shadow spell damage you deal