Death's Dance and Spirit Visage are both amazing on Illaoi.
Most of Illaoi's healing comes from her passive tentacle slapping heals, not from her AA lifesteal: that's why Hydra sucks for her.
Death's Dance heals you off all damage you deal, including your Tentacle Lashes, your W % max HP damage, your vessel damage, and your ultimate. Plus, it's basically 15% damage reduction, because once Illaoi is at low HP - her self-healing will help keep her afloat. Combined, Death's Dance might be one of the best
Defensive items for Illaoi - despite having no HP, Armor, or MR on it. Mandatory IMO.
Throw a Spirit Visage on that, and she's immortal once she's in melee range
Edit: So here's my question - has anyone played around with Duskblade Illaoi? It would only apply Nightfall on her basic attacks, which would proc off her W - but not her tentacles etc - and basically any time Illaoi lands a W is kill-time: all the more so if she had a Duskblade (plus, if you thought running away from Vessel was bad, how doomed would you be knowing you have Nightfall on you!)
Plus obviously Illaoi loves flat AD, and Duskblade is (one of?) the most efficient items for flat AD. Plus, 5% movement speed on Illaoi is always good.