I made some thoughts in which decks Idarran can add most value and did a small ranking.
1)
Arachas Swarm +
Caranthir Ar-Feiniel + Kikimore Queen +
Arachas Behemoth +
Endrega Queen +
Endrega Larva + many organic specials (+Defender)
+ Probably Tripple Kiki can be the most powerful point machine but you must be lucky with your draws (maybe add Oneiromancy and/or Naglfar to enhance draw likelihood). In case this combo does not work Endrega Larvaes, leader ability and so will still add significantly value
- High likelihood of overflooding your board.
Glustyworp can help to get some space, if you use him for Kiki Queen his row is basically full.
2) Fruits of Ysgith +
Katakan +
Endrega Larva +
Nekker + Koschey (Defender)
+ Can intensely increase your thrive tools, Katakan works both as a thrive-triggerer, thriver-getter and spawner
- Except the leader there are only 6 cards out of 25 which benefit from Idarran
3)
Congregate +
Passiflora + Adriano the Mink + Firesworn swarmer + Ivar the Hook (+Defender)
+ Get a lot engines out of Idarran
- Might have some problems with generating enough coins and activating Passiflora, Risk of overflooding your board; loose valuable devotion (Ulrich and Jacques)
4) Overhelming hunger +
Haunt +
Katakan +
Celaeno Harpy +
Endrega Larva +
Endrega Eggs +
Nightwraith + Idr (+Defender)
+ Having various units whch benefit from Idarran
- Risk of overflooding your board
5)
Deadeye Ambush +
Feign Death +
Vernossiel +
Ele'yas +
Half-Elf Hunter +
Waylay +
Scoia'tael Neophyte +
Aen Seidhe Sabre +
The Great Oak +
Aelirenn +
Isengrim Faoiltiarna +
Yaevinn
+The additional elf tokens can add intense value via Eleyas (+5 Deploy value; +1 additional elf token), Yaevinn, The Great Oak, Aelirenn, Isengrim and Vernossiel
- To my mind biggest risk of board overflooding and negative synergies with elfs like
Vrihedd Vanguard and
Vernossiel's Commando
6)
Ursine Ritual +
Cerys an Craite +
Svalblod Totem +
Harald Houndsnout + Lippy Goldmund
+ Svalblod Totem can be 18 for 10; nice carryover with Cerys
- very limited amount of target cards (maybe add some neutral spawners)
7)
Mobilization +
Queen Adalia +
Forbidden Magic +
Kaedweni Revenant + Redanian Archer (+ Operateur) (+Siege package)
+ Can be powerful as an additional Redanian archer can help to to have enough targets for Revenants
- Current Revenant decks are doing a good job, likely that deck gets less proactive if you have to spend one turn for Idarran, vulnerable to row punishment