Why didn’t Ravana use the Chandrahasa sword against Lord Rama?
Yes. Ravana used his sword against Jatayu. Ravana forcefully abducts Seetha in his air-chariot also known as Pushpaka Vimana. Finally, Seetha sees Jatayu, the eagle, and asks him to narrate her abduction to Rama. Jataayu confronts Ravana on hearing the wailing of Seetha. He boldly forestalls Ravana and his air-chariot in the sky itself, and as an elderly being he reviles Ravana from the viewpoint of righteousness befitting to kings, but of no avail.
Then the combat between Ravana and Jatayu takes place in mid-air. Ravana uses bow and arrows to fight with Jatayu. This is how sage Valmiki explains the combat between Ravana and Jayayu
ततो नालीक नाराचैः तीक्ष्ण अग्रैः च विकर्णिभिः |
अभ्यवर्षत् महाघोरैः गृध्र राजम् महाबलः || ३-५१-४
tato nālīka nārācaiḥ tīkṣṇa agraiḥ ca vikarṇibhiḥ |
abhyavarṣat mahāghoraiḥ gṛdhra rājam mahābalaḥ || 3-51-4
Then the great-mighty Ravana incessantly stormed the lord of eagles Jataayu, with arrows that have excruciating and highly dreadful arrowheads, like tubular arrows, iron arrows, and with arrows that have crescentic arrowheads.
But Jatayu sustained those ferocious arrows. He broke the bow and armour of Ravana during the battle.
ततो अस्य सशरम् चापम् मुक्ता मणि विभूषितम् |
चरणाभ्याम् महातेजा बभंज पतगोत्तमः || ३-५१-१०
tato asya saśaram cāpam muktā maṇi vibhūṣitam |
caraṇābhyām mahātejā babhaṃja patagottamaḥ || 3-51-10
That unexcelled bird of high refulgence then shattered Ravana's bow which is decorated with pearls and gemstones, and on which an arrow is admitted targeting the eagle, just by the pair of his bare feet.
तत् च अग्नि सदृशम् दीप्तम् रावणस्य शरावरम् |
पक्षाभ्याम् च महातेजा व्यधुनोत् पतगेश्वरः || ३-५१-१४
tat ca agni sadṛśam dīptam rāvaṇasya śarāvaram |
pakṣābhyām ca mahātejā vyadhunot patageśvaraḥ || 3-51-14
That highly resplendent lord of birds also blasted off the armour of Ravana, which is glowing and flamelike in its flare, with both of his feet.
कांचन उरः छदान् दिव्यान् पिशाच वदनान् खरान् |
तान् च अस्य जव संपन्नान् जघान समरे बली || ३-५१-१५
kāṃcana uraḥ chadān divyān piśāca vadanān kharān |
tān ca asya java saṃpannān jaghāna samare balī || 3-51-15
Mighty Jataayu also knocked off the ghost-faced mules yoked to the chariot of Ravana which are covered in golden armours and tantivy in fastness.
अथ त्रिवेणु संपन्नम् कामगम् पावक अर्चिषम् |
मणि सोपान चित्र अंगम् बभंज च महारथम् || ३-५१-१६
atha triveṇu saṃpannam kāmagam pāvaka arciṣam |
maṇi sopāna citra aṃgam babhaṃja ca mahāratham || 3-51-16
Then that great-chariot which is flourishing with three bamboos from chassis to yoke, and which traverses just by its steersman's wish, and which is crafted fantastically with gem-studded body and stairs, or, whose wheels are crafted with gold and gemstones, and which in its flare is like a Ritual-fire, Jataayu splintered down even that chariot of Ravana.
पूर्ण चन्द्र प्रतीकाशम् छत्रम् च व्यजनैः सह |
पातयामास वेगेन ग्राहिभी राक्षसैः सह || ३-५१-१७ pūrṇa candra pratīkāśam chatram ca vyajanaiḥ saha |
pātayāmāsa vegena grāhibhī rākṣasaiḥ saha || 3-51-17
Jataayu swiftly collapsed the parasol of Ravana which in shine is mirroring the full moon, along with the regalia of white-royal-fur-fans, together with the demons handling them for fanning Ravana.
सारथेः च अस्य वेगेन तुण्डेन च महत् शिरः |
पुनः व्यपाहरत् श्रीमान् पक्षिराजो महाबलः || ३-५१-१८
sāratheḥ ca asya vegena tuṇḍena ca mahat śiraḥ |
punaḥ vyapāharat śrīmān pakṣirājo mahābalaḥ || 3-51-18
Again that highly energetic and imposing king of birds pecked off the robust head of the charioteer of Ravana with beak.
स भग्न धन्वा विरथो हत अश्वो हत सारथिः |
अंकेन आदाय वैदेहीम् पपात भुवि रावणः || ३-५१-१९
sa bhagna dhanvā viratho hata aśvo hata sārathiḥ |
aṃkena ādāya vaidehīm papāta bhuvi rāvaṇaḥ || 3-51-19
Now Ravana who is with his devastated bow, devoid of chariot, dispatched are his horses and departed is his charioteer, and such as he is, he on grabbing Vaidehi by her torso, or, placing her on the lower end of his torso, jumped to earth.
Jatayu advises Ravana not to abduct Seetha. He warns that abducting Seetha will ruin him(Ravana) and his whole clan. But Ravana doesn't listen to him. Again an encounter takes place between both of them. Jatayu tears the back of Ravan with his sharp beak and claws. Jatayu rips off the ten left arms of Ravana with his beak inorder to release Seetha from Ravana. But Ravana's arms instantaneously ricocheted from his body.
Now Ravana uses his sword against Jatayu who was fighting with him for the sake of Rama.
तस्य व्यायच्छमानस्य रामस्य अर्थे अथ रावणः |
पक्षौ पादौ च पार्श्वौ च खड्गम् उद्धृत्य सो अच्छिनत् || ३-५१-४२
tasya vyāyacchamānasya rāmasya arthe atha rāvaṇaḥ |
pakṣau pādau ca pārśvau ca khaḍgam uddhṛtya so acchinat || 3-51-42
Ravana brandishing his sword at Jataayu, who is revolting for the sake of Rama, hewed off both of his wings, sides, and feet.
स छिन्न पक्षः सहसा रक्षसा रौद्र कर्मणा |
निपपात महा गृध्रो धरण्याम् अल्प जीवितः || ३-५१-४३
sa chinna pakṣaḥ sahasā rakṣasā raudra karmaṇā |
nipapāta mahā gṛdhro dharaṇyām alpa jīvitaḥ || 3-51-43
When that demon of cruel actions has ruptured his wings that colossal eagle Jataayu immediately fell down to earth with a lessened life.
By above verses it is clear that Ravana used his sword against Jatayu.
Lord Shiva said to Ravana : ‘You should not disregard this sword. If you do, it will undoubtedly return to me.’
Ravana used the sword to kill/harm Jatayu who was forestalling him in the abduction of Seetha. This is clearly adharma.
After this incident, there is only one incident where Ravana uses a sword but neither the name is mentioned nor the origin of it.
उत्प्लुत्य गुणसम्पन्नं विमलाम्बरवर्चसं |
निष्पपात स वेगेन सभायाः सचिवैर्वृतः || 6-92-39
रावणः पुत्रशोकेन भृशमाकुलचेतनः |
संक्रुद्धः खड्गमादाय सहसा यत्र मैथिली || 6-92-40
The enraged Ravana, who was very much agitated in mind because of the grief for his son's death, springing up suddenly and taking his excellent sword, having the dazzle of a clear sky, sallied forth from the assembly hall along with his ministers swiftly and quickly to the place where Seetha was.
He thought of killing Seetha using his sword but got pacified later.
It was not explicitly mentioned that this sword is Chandrahasa.
Conclusion: The sword used against Jatayu should possibly be Chandrahasa. The sword would have returned to Lord Shiva after this Jatayu incident. As the sword has returned, there is no mention of him using a sword against Rama in the final battle. We cannot dismiss claim of using a sword in the battle saying it is a distance battle. Sword was used against Sugriva by the demon Virupaksha.
Note: “The question: Why didn’t Ravana use the Chandrahasa sword against Lord Rama?” is licensed by Stack Exchange Inc (https://hinduism.stackexchange.com/); user contributions licensed under CC BY-SA.