A "doomstack" is a large and powerful army that comprises the majority of a nation's available combat power, using elite troops and significant mage support along with a significant commitment of gems for battlemagic and battlefield-wide buffs.
They are used whenever a player wants to put forth maximum effort to win a symmetric battle against another large army.
They are often countered by asymmetric means by nations that cannot fight them head on, so designing a doomstack to anticipate and respond to these asymmetric counters is important. Doomstack counters include:
Battlefield-wipe spells like Foul Vapors or Earthquake, which can do great damage even if their caster dies
Players can ensure that their army is resistant to the most common ones using spells like Serpent's Blessing or Mass Flight, include elements that can disable a suicide caster quickly, and plan for castle sieges carefully
Remote attack spells like Murdering Winter, Flames from the Sky, or Leprosy
Players can mitigate remote attacks by only assembling a doomstack when it is needed to fight, using resilient troops wherever possible, putting resistance gear on vulnerable mages, masking it with
Cauldron of the Elven Halls, feigning doomstacks at other times and places with false army items, and protecting it when possible using
domes.
Specially-designed supercombatants tailored to be immune to that army's capabilities
Ignoring the doomstack and using smaller forces to raid an opponent's other provinces, so that the assets gained by the doomstack are less valuable than the assets lost to hostile raiding