Madara Uchiha fell silent. If he could, he would have killed Uchiha Yi outright.
But Yi had the Flying Thunder God Technique. One slip, and he might escape. Then word that Madara was still alive would get out, and that would be disastrous for his plans.
As for threatening Yi with someone he cared about, Madara knew the Uchiha. If something happened, Yi might awaken the Mangekyō Sharingan, which would make him even harder to deal with. Madara did not have the Rinnegan yet.
Of course, Madara refused to admit he was afraid of Yi. The real problem was his condition. Like Mito Uzumaki, he still had one fight left in him, but afterward, he would likely be finished.
Madara would not gamble on the life force within the Demonic Statue of the Outer Path keeping him alive indefinitely.
After a long hesitation, Madara said, "Find a way to get the other villages to send troops against Konoha too. Kirigakure in particular. They can stop their infighting and go straight to war with Konoha. Have them throw everything they have at it.
"Even if they can't weaken Yi himself, they must weaken Konoha as a whole."
Madara had considered having the other villages' Jinchūriki master their Tailed Beasts and use them against Yi, but that would defeat the purpose.
They would need to collect those Tailed Beasts to carry out the Eye of the Moon Plan. Making the Jinchūriki stronger would only hurt their plans, unless Madara died first and Nagato revived him with the Rinnegan.
But Nagato could not use Rinne Rebirth yet. Reviving Madara would take an enormous amount of power.
Madara even suspected Nagato could not revive him at all unless he became the Ten-Tails Jinchuriki or acquired a vast quantity of Tailed Beast Chakra.
Reviving an