Something similar to what was done with the Amurites in the Amurite+ module would work well for the Sidar, rather than going nuts with terrain altering effects or special counters; not that these things wouldn't be fun, just that they become decidedly less unique if every civ uses one.
The Sidar are thematically a defensive/builder-type that uses isolation, espionage, and assassination to defend their borders. The Sidar should receive significant improvements to their recon line units, special advantages for exploration and intelligence gathering, and additional defensive mechanisms. I would probably also give their leaders a quick review.
Here's what I suggest for the Sidar+ module:
Recon Line units:
UU Scout: Sidar Spy
* 0/2, cannot attack, starts with Sentry I, permanently hidden, can explore rival territory, can investigate cities, never becomes obsolete
UU Ranger: Umbral Knight
* 6/6, +25% bonus vs undead, can use bronze and iron weapons/poisoned blade promotion
Arcane Line units:
* Sidar mages, and any T3 Sidar unit or special disciple unit with Combat IV or higher, can summon a Phantasmal Host, but only while within Sidar cultural borders.
New Summon: Phantasmal Host
* 4/4, +1 mind aptitude, +1 shadow aptitude, starts with Illusion, Blur, Shadowwalk, 45% chance to withdraw from combat, temporary summon (1 turn duration without Summoner trait).
* Sidar casters may not purchase the Death I, II, or III upgrades. The Sidar may connect a death node for strategic purposes but they cannot use necromancy.
Disciple Line units:
UU Priest: Shadow Convoker (national unit - limit 4)
* buildable at sorcery
* 4/4, +1 mind aptitude, +1 shadow aptitude, starts with channeling II, shadow I, life I, divine, medic I + II, stealth
UU High Priest: Shadow Chancellor (national unit - limit 1)
* may only be upgraded from a level 6 or higher Shadow Convoker
* requires Force of Will (or Arcane Lore if you're feeling generous)
* 5/5, +1 mind aptitude, +1 shadow affinity, starts with channeling III, shadow I, life, I, divine, medic I, II, + III, stealth
The Idea:
The Sidar defense is based on a well developed Recon and Arcane line. The Umbral Knight UU almost keeps up with a Mithral-armed Champion unit; deployed en masse with Shock promotions these units can counter a melee heavy invader. A good % of Sidar units will be able to summon Phantasmal Hosts, lethal nightmares that demoralize and even kill intruders into the Sidar domain. Those that survive are the next to face the blades of Sidar Shadows.
The Sidar unique disciple units represent the Shadow Conclave, a Machiavellian council that concerns itself with the balance of power in Erebus. Their nefarious purpose is largely secret even in Sidar society where the Convokers are regarded as sages and luminaries, completely eternal and amoral -- the ideal for Sidar society. The Shadow Chancellor is Rathus Denmora's chief advisor. 1st amongst the Conclave, the Chancellor's identity is known only by the members of the Conclave and Rathus Denmora himself.
The Sidar unique disciple units and the Umbral Knight UU are intended to act as a thematic soft counter to undead heavy civilizations; and to ensure that the Sidar are a difficult civilization for the Scions to deal with. The introduction of a few unique disciple units also makes going Esus an easy decision for the Sidar.